Compendium of Brassica Diseases

Compendium of

Brassica Diseases

Compendium of Brassica Diseases Edited by S. Roger Rimmer Agriculture and Agri-Food Canada Saskatoon Research Centre, Saskatchewan, Canada

Vernon I. Shattuck Alta Mefa Drive Riverside, California

Lone Buchwaldt Agriculture and Agri-Food Canada Saskatoon Research Centre, Saskatchewan, Canada

The American Phytopathological Society

Front cover photograph: Oil seed rape, white cabbage, red cabbage, broccoli, cauliflower, daikon (lo bok), radish, watercress, Chinese cabbage, yu choy, baby pak choi, suey choy, Chinese kale (gai lan), broccoli raab (rapini), kale, turnip, and rutabaga. (Courtesy L. Buchwaldt and R. Underwood) Back cover photograph: Canola field in southwestern Ontario, Canada.

(Courtesy V. I. Shattuck)

Reference in this publication to a trademark, proprietary product, or company name by personnel of the U.S. Department of Agriculture or anyone else is intended for explicit description only and does not imply approval or recommendation to the exclusion of others that may be suitable.

Library of Congress Control Number: 2007923101 International Standard Book Number: 978-0-89054-344-3 Š 2007 by The American Phytopathological Society All rights reserved.

No portion of this book may be reproduced in any form, including photocopy, microfilm, information storage and retrieval system, computer database, or software, or by any means, including electronic or mechanical, without written permission from the publisher.

Copyright is not claimed in any portion of this work written by U.S. government employees as a part of their official duties.

Copyright is not claimed in any portion of this work written by employees of Agriculture and Agri-Food Canada. Š 2007 by Department of Agriculture and Agri-Food, Government of Canada Printed in the China on acid-free paper.

The American Phytopathological Society 3340 Pilot Knob Road St. Paul, Minnesota 55121, U.S.A.

Preface ernment publications for more information on locally adapted cultivars, use of registered fungicides, and proven cultural practices. Competent specialists in the area, such as extension plant pathologists or informed agricultural or horticultural suppliers, should also be consulted. The references after each section are a selection of contemporary and classical scientific papers, reviews, monographs, and conference proceedings. Because of space limitation, the compendium does not deal with damages caused by insect pests, except for insect vectors, which are described in the sections dealing with diseases caused by viruses and by mollicutes. Comments regarding the general usefulness of this compendium or omissions from it are welcome and should be directed to either APS PRESS or directly to the senior editor. With readers’ suggestions, future editions can be even more valuable.

The editors thank the many authors who helped prepare the text, as indicated on the following pages and at the end of the various sections. We also appreciate the cooperation of many individuals who allowed us to use their color plates, black-andwhite photographs, and line drawings, which greatly enhance the usefulness of the compendium. We appreciate the support of our financial sponsors, secretarial help of Kay Prince and Jodi Bernath, and graphic support of Ralph Underwood. Finally, we would like to thank the reviewers, Drs. S. Miller, E. Braun, D. Pink, and M. Dickson, for their most helpful suggestions for improvement of the Compendium of Brassica Diseases.

The purpose of the Compendium of Brassica Diseases is to provide a thorough, authoritative, and practical reference guide for people engaged in diagnosing and managing disease problems in brassica crops destined for vegetable markets, seeds, oil, and condiments the world over. The compendium is created for extension plant pathologists working in agriculture and horticulture and for other advisors in governmental departments, including individuals in regulatory agencies, as well as those in the private sector dealing with seed and fungicide retail and with disease scouting. The book is also useful for brassica crop growers, plant pathology teachers and students, research scientists, and plant breeders in countries where brassicas are grown.

Sections within the compendium were written with the aid of authors from different parts of the world with years of research experience on a particular disease, storage problem, non­infectious disorder, etc. The sections are organized into three parts. Part I explains the taxonomic and genetic relationships of the many crop species in the family Brassicaceae, followed by an account of various practices used in brassica crop production. Part II includes descriptions of the most important diseases caused by fungal, bacterial, viral, mollicute, and nematode pathogens. Part III describes noninfectious disorders that are caused by abiotic factors, and it also includes a section on storage problems. Sections in Parts II and III are generally organized alphabetically by the most common name of the infectious and noninfectious diseases. Suggested control measures described at the end of individual sections are purposely general and emphasize those principles that should not become obsolete. However, readers should consult local gov-

S. Roger Rimmer Vernon I. Shattuck Lone Buchwaldt

iii

Authors K. R. Barker Plant Pathology Department North Carolina State University Raleigh, NC

J. Fletcher Department of Plant Pathology Oklahoma State University Stillwater, OK

A. Bratsch Department of Horticulture Virginia Polytechnic Institute Blacksburg, VA

P. Gladders ADAS Boxworth, Cambridge, United Kingdom

L. Buchwaldt Agriculture and Agri-Food Canada Saskatoon, SK, Canada

R. C. Herner Department of Horticulture Michigan State University East Lansing, MI

R. Cerkauskas Greenhouse and Processing Crops Research Centre Agriculture and Agri-Food Canada Harrow, ON, Canada

C. B. Hill Department of Crop Sciences University of Illinois Urbana-Champaign, IL

J. M. L. Davies ADAS Lincs, United Kingdom

H. J. Hopen Department of Horticulture University of Madison Madison, WI

M. H. Dickson Department of Seed and Vegetable Science Cornell University Geneva, NY

A. J. Inman Defra Plant Health Division York, United Kingdom

G. R. Dixon Department of Horticulture University of Strathclyde Glasgow, Scotland, United Kingdom

Ă?. Mamula Department of Botany University of Zagreb Zagreb, Croatia

J. S. Dodson Arroyo Grande Research Station Arroyo Grande, CA

D. N. Maynard Golf Coast Research and Educational Centre University of Florida East Bradenton, FL

K. J. Doughty Bayer AG Agricultural Centre Monheim Leverkusen, Germany

H. A. McCartney (retired) Rothamsted Research Harpenden, United Kingdom

D. M. Eastburn Department of Plant Pathology University of Illinois Urbana, IL

R. H. Morrison Sakata Seed America Salinas, CA N. I. Nashaat Rothamsted Research Harpenden, United Kingdom

C. Eastman Centre for Economic Entomology Illinois Natural History Survey Champaign, IL

J. W. Noling Citrus REC University of California, Riverside Lake Alfred, FL

B. D. L. Fitt Rothamsted Research Harpenden, United Kingdom v

C. Olivier Agriculture and Agri-Food Canada Saskatoon, SK, Canada

S. B. Sterrett Eastern Shore Agriculture and Experiment Station Virginia Polytechnic Institute Painter, VA

D. P. Ormrod Department of Biology University of Victoria Victoria, BC, Canada

Y. Takanami Department of Applied Genetic and Pest Management Kyushu University Fukuoka, Japan

V. H. Paul Laboratory for Biotechnology and Quality Assurance University of Applied Sciences Soest, Germany

J. P. Tewari (retired) Department of Plant Science University of Alberta Edmonton, AB, Canada

G. A. Petrie (retired) Agriculture and Agri-Food Canada Saskatoon, SK, Canada

K. Topinka University of Alberta Edmonton, AB, Canada

S. R. Rimmer Agriculture and Agri-Food Canada Saskatoon, SK, Canada

J. Uyenaka Ontario Ministry of Agriculture, Food and Rural Affairs Ancaster, ON, Canada

H. Sanfaçon Agriculture and Agri-Food Canada Summerland, BC, Canada

C. G. J. van den Berg ICMS, Inc. Abbotsford, BC, Canada

V. I. Shattuck Alta Mefa Drive Riverside, CA

P. R. Verma (retired) Agriculture and Agri-Food Canada Saskatoon, SK, Canada

A. E. Simon Department of Biochemistry and Molecular Biology University of Massachusetts Amherst, MA

J. A. Walsh University of Warwick - Horticultural Research International Wellesbourne, Warwick, United Kingdom

J. Špak Institute of Plant Molecular Biology Biological Centre, Czech Academy of Sciences Ceske Budejovice, Czech Republic

S. I. Warwick Agriculture and Agri-Food Canada Ottawa, ON, Canada G. E. Welbaum Department of Horticulture Virginia Polytechnic Institute Blacksburg, VA

R. Stace-Smith (retired) Agriculture and Agri-Food Canada Vancouver, BC, Canada Z. Stefanac Department of Botany University of Zagreb Zagreb, Croatia

P. H. Williams (retired) University of Madison Madison, WI

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Contents Part I. Introduction

39 Rhizoctonia Diseases (Damping-Off, Wirestem, Brown Girdling Root Rot, Crater Rot, Head Rot) 41 Ring Spot 43 Sclerotinia White Mold (Cabbage Drop, Stem Rot, Watery Soft Rot) 47 Verticillium Wilt 50 White Leaf Spot 54 White Rust 56 Yellows (Fusarium Wilt) 58 Diseases Caused by Bacteria 58 Bacterial Leaf Spot 59 Bacterial Soft Rot 60 Black Rot 62 Diseases Caused by Mollicutes 62 Aster Yellows 64 Brittle Root of Horseradish 66 Diseases Caused by Viruses 66 Beet Western Yellows (Turnip Yellows Luteovirus) 66 Cauliflower Mosaic 68 Cucumber Mosaic 68 Radish Mosaic 70 Ribgrass Mosaic 71 Turnip Crinkle 72 Turnip Mosaic 73 Turnip Yellow Mosaic 75 Other Virus Diseases 75 Diseases Caused by Nematodes 76 Cyst Nematodes 78 Root-Knot Nematodes 81 Other Nematodes

2 Taxonomy and Genetic Relationships of Brassica Species 3 Brassica 3 Species Relationships and Origins of Crop Brassicas 3 Brassica oleracea 4 Brassica rapa 5 Brassica nigra 6 Brassica napus 6 Brassica carinata 6 Brassica juncea 6 Crambe abyssinica 6 Eruca vesicaria subsp. sativa (E. sativa) 6 Raphanus sativus 7 Production Management 7 General Considerations 8 Vegetable Crops 8 Cabbage 9 Cauliflower, Broccoli, and Brussels Sprouts 10 Kohlrabi 11 Greens 11 Asian Vegetable Brassica Crops 11 Asian Mustard Greens 11 Chinese Broccoli (Chinese Kale) 11 Chinese Cabbage 12 Root Crops 12 Horseradish 12 Radish 13 Turnip and Rutabaga 13 Oilseed Crops 13 Oilseed Rape (Canola, Rapeseed) 13 Culinary or Condiment Mustards

Part III. Noninfectious Diseases

Part II. Infectious Diseases

82 83 83 84

15 Diseases Caused by Fungi and Oomycetes 15 Alternaria Diseases (Black Spot, Gray Leaf Spot, Pod Spot) 18 Anthracnose 19 Black Leg (Phoma Stem Canker) 22 Black Root 24 Botrytis Gray Mold 25 Clubroot 28 Downy Mildew 31 Light Leaf Spot 35 Phytophthora and Pythium Damping-Off 35 Phytophthora Root Rot 37 Phytophthora Storage Rot 37 Powdery Mildew

84 85 85 85 86 86 87 87 87 88 88 89 89 vii

Air Pollution Disorders Related to Environmental Effects Black Speck of Cauliflower Bolting or Buttoning of Cauliflower, Broccoli, and Cabbage Cold Injury Drought Injury Growth Scars and Tissue Splitting Hail Injury Heat Tolerance in Broccoli and Cauliflower Hollow Stem Intumescence Mechanical Injury Edema Petiole Freckles (Gomasho) of Chinese Cabbage Riciness of Cauliflower Root Cavities of Daikon (Lo Bok) Salt Injury

89 89 90 90 91 91 93 93

Soil Compaction Problems Soil Moisture Injury Strangles (Windwhip) Winter Decline Syndrome Yellow Eye (Starring, Cat Eye) Genetic Abnormalities Herbicide Injury Herbicide Injury Due To Improper Timing or Rate

95 96 98 103

Herbicide Injury Due To Unintended Exposure Herbicide Injury Due To Carryover Nutritional Deficiencies Postharvest Disorders of Vegetable Brassicas

105 Glossary 111 Index

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Compendium of Brassica Diseases

Part I. Introduction

This compendium strives to present a balanced account of the diseases that occur on the broad spectrum of brassica crops worldwide. Brassicas, and other closely related cruciferous crops, are an extraordinarily diverse group of plant types that are widely cultivated throughout the world as leaf or root vegetable crops, for vegetable oil production, as fodder crops for livestock feeding, and as condiments and spices to add flavor to human diets. The cultivation of Brassica rapa and B. juncea dates back to ca. 1500 b.c. Historically, human consumption of brassica vegetables and vegetable oils was primarily concentrated in Asiatic countries, predominantly in the northern Indian subcontinent and China.

Brassica vegetables are well known to most people, and large quantities are consumed, especially in Asia and Europe. The cabbage types are widely grown in temperate parts of the world and are traditionally stored during the winter months either intact or fermented (e.g., sauerkraut and kimchi). The world production of both cabbage and cauliflower has increased during the last 20 years, particularly in China (Table 1). Some brassica vegetables are gaining popularity, especially broccoli, because of increasing evidence of their anticarcinogenic properties, owing in part to the presence of glucosinolates and other antioxidant compounds. Other brassicas are preferred for their distinctive hot taste, related to the presence of specific glucosinolates, which are enzymatically hydrolyzed by myrosinase when cells are ruptured to release isothiocyanates and other sulfur­-­containing compounds.

Important changes in the quality of seed oil and the residual meal, involving modification of the fatty acid composition (elimination of erucic acid) and reduction of the glucosinolate content in the meal, together with increasing world demand for vegetable oils, have resulted in a dramatic increase in the production of oilseed rape or canola in Australia, Canada, China, Europe, and the northern states of the United States. Spring oilseed rape is now well established as a viable economic crop in many of the northern states of the United States and is also being evaluated as a winter crop in some of the southern states.

Total world production of oilseed rape is currently now in ex-

cess of 40 million metric tons (Table 1). Oilseed rape accounts for more than 10% of the total oilseed production and 15% of the world’s total edible vegetable oil production. In terms of world annual vegetable oil production, brassica oil is second only to soybean oil.

B. oleracea includes the major brassica vegetables, e.g., cabbage, cauliflower, broccoli, Brussels sprouts, kale, and kohlrabi.

It generally has a slow, steady growth habit with a large capacity for nutrient storage. The species usually requires substantial vernalization to initiate flowering, although annual types do occur (e.g., Chinese kale). The wild forms of this species are usually perennial.

B. rapa includes both leafy and root vegetable types (e.g., Chinese cabbage, pak choi, and turnip) and oilseed types (e.g., turnip rape, toria, yellow sarson, and brown sarson). It is adapted to cool environments and has a high relative growth rate. Both spring and winter annual types are cultivated, and the most cold­-­hardy cultivars of oilseed brassicas occur within this species.

B. juncea is an oilseed species grown worldwide. It is well adapted to drier conditions and is also relatively fast maturing.

On the Indian subcontinent, nonvernalization types of B. juncea are cultivated during the cool, moist winter season. In Canada, B. juncea is grown for condiment mustard, and recently, canola quality (low erucic acid and low glucosinolate contents) has been developed for oilseed production in drier regions where B. napus is not adapted. Oil from both species is now marketed as canola oil without any species distinction. Many leafy vegetable types of B. juncea, often with a pungent taste, are produced in Asia and many other parts of the world. B. nigra was formerly cultivated for mustard condiment seeds, but it has largely been replaced by cultivation of B. juncea and Sinapis alba.

In cool, temperate climates with good moisture levels, B. napus is preferred for oilseed production and is more productive compared with B. juncea. Most of the land area cultivated to oilseed brassicas in Europe and China is sown to winter oilseed rape. However, when winter conditions are too cold, summer forms of B. napus or winter and summer forms of turnip rape

1

(B. rapa) are grown. This species is also cultivated for its root, the rutabaga.

The genetic relationships among Brassica spp. and other closely related species are well understood. In the 1930s, ­Morinaga proposed that B. nigra, B. oleracea, and B. rapa are the primary species and that B. carinata, B. juncea, and B. napus are amphidiploids resulting from hybridization between corresponding pairs of the primary species. These relationships were confirmed by U, who succeeded in the artificial synthesis of B. napus from crosses between the diploid species B. rapa and B. oleracea. Consequently, the diagram showing the genomic relationships among the brassicas is often referred to as the triangle of U (Fig. 1). Synthesis of B. juncea and B. carinata has subsequently been accomplished by the interspecific hybridization between B. nigra and B. rapa or B. oleracea, respectively. Thus, the brassica crops (including radish) are closely related genetically, and it is not an impossible task to transfer desirable traits from one species to another.

Also, because of their close relationship, the brassica species have many diseases in common. However, even a cursory reading of this compendium demonstrates that there are also differences in susceptibility of brassicas to pathogen species. The large increase in global production of both vegetable and oil-

seed brassicas has been associated with new disease problems, some of which were previously unimportant. In Australia, the oilseed brassica industry was destroyed in its infancy because of the high severity of black leg. The industry was renewed recently, following the development of cultivars with resistance to this disease. A significant body of research on brassica diseases during the past 20 years has largely been directed at oilseed brassicas. The genetic similarity of brassicas to the wild cruciferous plant Arabidopsis allows us to believe that significant improvements in disease resistance will be achieved in the future as a better understanding of resistance genes, gleaned from genomic and molecular biology studies in Arabidopsis, is applied to brassica species.

This compendium describes diseases associated with the four major species, B. oleracea, B. rapa, B. juncea, and B. napus, which are cultivated for either vegetable, fodder, or oilseed purposes. In addition, diseases of other species of the family Brassicaceae, including radish, Raphanus sativus; mustards, B. juncea, B. nigra, B. carinata, and S. alba; and condiments, e.g., horseradish (Armoracia rusticana), are also included. Because of the wide range of crops within the family Brassicaceae, crop species are most frequently referred to as brassica crops in this compendium, even though, in some cases, they may be species from a different genus. The term “crucifers” is most frequently used in reference to wild and weedy species of the family Brassicaceae. Because of the diversity of species and plant types, the compendium begins with a section on taxonomy and on current production practices of the main brassica crops. In the following sections, fungal, bacterial, mollicute, viral, and nematode diseases are described in alphabetical order. A section is also devoted to disorders associated with noninfectious causes.

Selected References

Fig. 1. Genomic relationships (capital letters) between the three diploid Brassica species, B. nigra, B. oleracea, and B. rapa, and the amphidiploid derived species, B. carinata, B. juncea, and B. napus. n = The haploid chromosome number. (Adapted from U, 1935)

Downey, R. K., and Rimmer, S. R. 1993. Agronomic improvement in oilseed brassicas. Adv. Agron. 50:1­- ­66.

FAOSTAT. Agriculture. Agricultural production: Crops primary. http:// apps.fao.org/page/collections?subset=agriculture.

Gómez­- ­Campo, C., ed. 1999. Biology of Brassica Coenospecies. Elsevier Science B.V., Amsterdam.

Kimber, D., and McGregor, D. I. 1995. Brassica Oilseeds­—­Production and Utilization. CAB International, Wallingford, Oxon, U.K. Morinaga, T. 1934. Interspecific hybridisation in Brassica. VI. The cytology of F1 hybrids of Brassica juncea and B. nigra. Cytologia 6:62­- ­67.

Tsunoda, S., Hinata, K., and Gómez­- ­Campo, C. 1980. Brassica Crops and Wild Allies: Biology and Breeding. Japan Scientific Societies Press, Tokyo.

U, N. 1935. Genome analysis in Brassica with special reference to the experimental formation of Brassica napus and peculiar mode of fertilization. Jpn. J. Bot. 7:389­- ­452.

(Prepared by S. R. Rimmer and L. Buchwaldt)

Taxonomy and Genetic Relationships of Brassica Species The Brassicaceae (= Cruciferae) or mustard family has several important crop and weed species. The taxonomic hierarchy of the family provides a framework for identifying relatives and predicting relationships for brassica crop species (Table 2). The family is composed of approximately 350 genera and 3,500 species. The tribe Brassiceae is considered a natural group, having originated from a common ancestor. The tribe contains 52 genera and is distinguished by its two­-­segmented fruit and unique arrangement of the cotyledons or first leaves in the seeds. The tribe is further divided on the basis of fruit length into six subtribes. The genus Brassica is one of nine 2

core genera in the subtribe Brassicinae. Recent molecular studies and hybridization data do not support the separation of the subtribe Brassicinae from the subtribes Raphaninae and Moricandiinae. The species are further grouped into cytodemes or crossing groups (discussed below).

Extensive hybridization studies have been conducted among the brassica crop species and their wild relatives through natural, hand­-­pollinated, and artificial crosses. On the basis of chromosome number and crossing ability, Harberd in 1972 defined the Brassica coenospecies as the “group of wild species sufficiently related to the six cultivated species of Brassica to be

which are diploids, Brassica nigra (n = 8, genome BB), B. oleracea (n = 9, genome CC), and B. rapa (n = 10, genome AA), and three of which are amphidiploid derivatives, B. carinata (n = 17, genome BBCC), B. juncea (n = 18, genome AABB), and B. napus (n = 19, genome AACC). The cytogenetic relationships of the three amphidiploid species proposed in 1935, and known as the triangle of U (Fig. 1), have been confirmed by chromosome pairing and artificial synthesis of the amphidiploids, nuclear DNA content, DNA analysis, and use of genome­­specific markers. With respect to the three diploid taxa, recent DNA analyses, including both nuclear and chloroplast restriction site data, suggest separate evolutionary pathways: B. rapa and B. oleracea (including wild CC­-­genome species) assigned to one group, with Diplotaxis erucoides (L.) DC. (n = 7) or a close relative as the primary progenitor species, and B. nigra assigned to a second group, with Sinapis arvensis L. or a close relative as the primary progenitor species.

Based on the distribution of its wild species, the genus Brassica probably originated in the Mediterranean–Middle Eastern area.

A secondary center of origin and of differentiation of the species B. rapa and B. juncea appears to be China. Since their introduction into China thousands of years ago, these two species have been changed significantly in form, structure, and productivity by domestication. As a result of the allogamous breeding system in the genus Brassica, there is a large amount of morphological variability in the many subspecies, botanical varieties, and cultivar groups in B. oleracea, B. rapa, and B. juncea. Numerous parallel vegetable forms have been selected, and the three species have differentiated historically along similar lines. potentially capable of experimental hybridization with them.”

The coenospecies corresponds closely to the taxonomic subtribe Brassicinae, with the inclusion of the genera Raphanus, Enarthrocarpus, Moricandia, and Orychophragmus. Forty­­five diploid cytodemes or crossing groups and six amphidiploid taxa are described for the coenospecies. The cytodeme is characterized by a single, diploid chromosome number and can be defined as a group of fully interfertile taxa, usually isolated geographically and often given specific rank in different parts of the range. A given cytodeme is usually not fertile with other cytodemes through ordinary sexual means.

Brassica The old world genus Brassica includes about 35 species of mostly annual herbs, with some perennial herbs and small shrubs. The crop brassicas comprise six economically important species with great genetic and morphological diversity.

These plants yield edible roots, stems, leaves, buds, flowers, and seeds. In addition, some of the types are used as forage, sources of oil, or ornamentals.

Separating the intergrading variants that different authorities have recognized is difficult, even for specialists. Several systems of formal Latin nomenclature have been developed for the genus Brassica. The work of L. H. Bailey on classification of cultivated Brassica spp. in the 1920–1940s, although dated, remains one of the most authoritative. This work has been incorporated into a synoptic taxonomic treatment by Hanelt in Schultze­-­Motel (1986), which is generally followed here because it is the most complete modern work available.

Species Relationships and Origins of Crop Brassicas The origin, evolution, taxonomy, and genomic relationships of the crop brassicas have been studied extensively. Cultivated brassicas are represented by six interrelated species, three of

Brassica oleracea Brassica oleracea, generally designated as cole crops, has a great diversity of morphotypes that have been given various varietal rank, subspecific rank, or both (Table 3). The crop was domesticated during the first millennium b.c. B. oleracea is a member of the CC­-­genome complex or B. oleracea cytodeme. It includes a number of interfertile Mediterranean species (B. cretica Lam., B. hilarionis Post, B. incana Ten., B. insularis Moris, B. macrocarpa Guss., B. montana Pourr., B. ­rupestris Raf., and B. villosa Biv.), wild B. oleracea from coastal areas of western Europe, and B. bourgeaui (Webb) Kuntze from the Canary Islands.

The most important crops in B. oleracea (varieties in parentheses; Table 3) are • kales (var. viridis, var. costata, var. medullosa, and var. sabellica), including kitchen kale, green kale, dwarf ­Siberian kale, marrow­-­stem kale, collards, and tronchuda, which develop a strong main stem and are used for their edible foliage; • branching-bush kales (var. ramosa), including thousand­­head kale and perpetual kale, which were formerly much cultivated for their edible foliage; • cabbages (var. capitata and var. sabauda), including headed cabbages and savoy cabbage, which form heads from the apical bud and consist of tightly packed leaves; • Brussels sprouts (var. gemmifera), which has axillary buds forming edible heads of tightly packed leaves; • kohlrabi (var. gongylodes), which is cultivated for its aboveground thickened stem; • inflorescence kales (var. botrytis and var. italica), including cauliflower, broccoli, and sprouting broccoli, which are cultivated for their thickened edible floral meristems (cauliflower curd) or inflorescences (buds); and • Chinese kale (var. alboglabra), which is a cultivated white­-­flowering crop grown in China, generally assumed to be an ancient import from the Mediterranean region and often treated as a separate species, B. alboglabra.

3

Various hypotheses have been proposed to account for the origins of the different cultivated types. These include a single origin of all types from wild B. oleracea from western Europe and triple and even multiple origins involving related wild species of the complex. Recent molecular studies support a monophyletic origin for the cultivated morphotypes of B. oleracea from a progenitor that was similar to wild B. oleracea and B. alboglabra. This is consistent with morphological evidence that suggested that the earliest cultivated B. oleracea was probably a leafy kale from which the other cultivated types originated. Molecular evidence, i.e., occurrence of specific markers for B. insularis and B. incana in certain kale types, suggests that selective introgression from other wild CC­- ­genome cytodeme members may have contributed to the variability of cultivated B. oleracea.

Brassica rapa The species names Brassica rapa and B. campestris L. are used interchangeably in the literature, resulting in much confusion. B. rapa and B. campestris were described by Linnaeus 4

in 1753 to indicate turnip and wild, weedy plants, respectively.

The two taxa are interfertile and were first combined in 1833 by Metzger under the name B. rapa, which according to the International Rules of Plant Nomenclature, makes this the correct name for the species. B. rapa is a very polymorphic species and contains many crops that have been domesticated over a long period in Europe as well as in Asia. Little is known about its true existence in the wild; plants found under natural conditions seem to be escapes from cultivation (subspecies sylvestris).

B. rapa is most closely related to B. oleracea and both have arisen from ancestral members of the C­-­genome cytodeme.

The most important crops in B. rapa (subspecies in parentheses; Table 3) are • vegetable turnip (subsp. rapa); • fodder turnip (subsp. rapa), which forms a leaf rosette, a turnip, or both; • turnip rape (subsp. oleifera) and toria (subsp. dichotoma), which are black seeded with annual spring and biennial winter types and are used for oil extraction; • yellow sarson (subsp. trilocularis), which is a yellow­­seeded annual and is used for oil extraction; • Chinese cabbage (subsp. pekinensis), which is an Asiatic heading vegetable with winged petioles;

• pak choi (subsp. chinensis), which is a Chinese nonheading leaf vegetable with fleshy, but not winged, petioles; • mizuna, mibuna, komatsuna, or leaf turnip (subsp. nipposinica), which is an Asiatic nonheading leafy vegetable with many tillers and either pinnate (mizuna) or entire (mibuna) leaves; • broad­-­beaked mustard or Chinese savoy (subsp. narinosa), which is an Asiatic nonheading leafy vegetable with a flat rosette of many small leaves; and • broccoletto, broccoli raab, or rapini (formerly treated as B. ruvo, assigned to subsp. oleifera), which is a European vegetable with an enlarged, compact inflorescence.

Most of the above­-­mentioned crops have originally been described as separate species (Table 3), but crossing studies have shown that they readily intercross and, hence, belong to the n = 10 B. rapa cytodeme. Members of the complex are now generally treated taxonomically as subspecies of B. rapa (Table 3).

Various data (morphology, geographic distribution, isozymes, and nuclear restriction fragment length polymorphisms) have indicated a division of B. rapa into two main groups, perhaps corresponding to two independent centers of origin. The primary center is Europe and includes turnip and turnip rape, from which Asian sarson and toria types were derived. The second center is in China and contains the various Asian vegetables in-

dicated above. The relationships and origins of the latter group have been partially clarified by molecular studies. For example, molecular data supported the proposed origin of subspecies pekinensis in central China by hybridization between subspecies rapa and chinensis rather than its origin from subspecies chinensis alone.

Brassica nigra Brassica nigra, or black mustard, once widely grown as a condiment mustard, has largely been replaced by B. juncea.

It is still grown as a condiment mustard in parts of Asia. Although little information is available, the occurrence of landraces in Europe, the Mediterranean, and the Ethiopian plateau indicates that B. nigra probably originated in central and southern Europe. It is presumed to have been introduced into India relatively recently. As indicated earlier, B. nigra has evolved separately from the other two diploid Brassica spp., and numerous data sets (cytological, isozyme, nuclear, and chloroplast DNA restriction site data) have suggested a closer genetic relationship to the genus Sinapis, particularly the weed species S. arvensis L. (n = 9), than to B. rapa and B. oleracea.

5

Brassica napus This crop is of comparatively recent origin and wild populations have not been found. It is generally accepted that Brassica napus originated in southern Europe or the Mediterranean region, where the ranges of the two parental taxa B. rapa and B. oleracea overlap. Recent reports have provided evidence for multiple polyploid origins of B. napus, including crosses of B. rapa with B. oleracea and of B. rapa with one of the wild C­-­genome relatives, B. montana Pourr.

There are various classifications for the cultivated material and various interpretations of their relationship to the wild species. Two subspecies are generally recognized: the bi­ennial vegetable rutabaga or swede (subspecies rapifera, includes variety napobrassica) and an annual oilseed or fodder crop (subspecies napus or subspecies oleifera). A third subspecies, pabularia or leaf rape, is described as an annual with dissected leaves and as possibly the original amphidiploid type giving rise to the other two subspecies. Hanelt has included it in subspecies napus.

Brassica carinata Brassica carinata, or Abyssinian mustard, is both an oilseed and a vegetable crop in Ethiopia with little differentiation into various crop types. Although wild types have not been located, it is believed to have originated in the Ethiopian plateau of northeastern Africa as a cross between wild­-­growing B. nigra and cultivated kalelike forms of B. oleracea.

Brassica juncea Brassica juncea, or Indian or brown mustard, is grown in North America and Europe for condiment use, on the Indian subcontinent for seed oil, and in the Far East as a vegetable.

Because of ecogeographic variation and human selection, a number of morphologically distinct forms are available, including oleiferous, semioleiferous, rapiferous, and leafy types. Infraspecific variants are not formally given by Hanelt, although many are recognized in the Asian literature (Table 3).

There is some uncertainty as to the probable center of origin of B. juncea. It most likely originated in the Middle East or western Asian region, based on geographic sympatry of the parental taxa, B. nigra and B. rapa, and on the presence of wild­-­growing B. juncea in this area. Other hypotheses suggest Asiatic origins with the center of major diversity in China. It seems likely that B. juncea may have arisen more than once as a result of hybridization, similar to that revealed for B. napus.

Indeed, recent molecular studies have suggested more than one origin for each of the three B. juncea varieties examined.

tigated in many areas of the world, including the midwestern United States, the Netherlands, and Canada.

Eruca vesicaria subsp. sativa (E. sativa) The genus Eruca, an old world genus of the tribe Brassiceae, is composed of four species that are native to the Mediterranean region. One taxon is cultivated, E. vesicaria subsp. sativa (frequently referred to as E. sativa). The subspecies sativa (n = 11, E genome) is an annual herb that has been cultivated since ancient times as a leafy vegetable (rocket or arugula), either for salad (in the Mediterranean and North America) or as a cooked green (in Italy). It is also grown as a cold­-­weather oilseed crop to produce jamba oil in Asia, mainly in India but also in Pakistan and Afghanistan. The seed oil is used as an illuminant, a lubricant, hair oil, and a vesicant and for massage and pickling.

The species vesicaria occurs in the Mediterranean, whereas the subspecies sativa has been introduced and naturalized in many areas of the world. In some regions, such as in Mexico, naturalized populations are abundant and serious weeds.

Raphanus sativus The genus Raphanus, an old world genus of the tribe Brassiceae, is composed of two species: radish, R. sativus (n = 9, R genome), and wild radish, R. raphanistrum L. (n = 9). Radish has been cultivated for thousands of years and was grown extensively in ancient Egypt. R. sativus is not known in the wild, except for escapes forming weedy, naturalized populations. There is some controversy as to the probable center of origin of R. sativus. It most likely originated in the Middle East or western Asian region, possibly from R. raphanistrum; although other suggestions indicate Asiatic origins with a center of major diversity in China.

Several classes of radish have been selected through domestication (Table 3). The most important crops in R. sativus (varieties in parentheses; Table 3) are • small radish (var. sativus and var. radicula), which is grown for its edible root; • black or large radish (var. niger and var. longipinnatus), which is grown for its roots, leaves, and young seed pods (believed to be the oldest type); • mougri, rat­-­tail, or aerial radish (var. mougri and var. caudatus), which is grown primarily for its edible young seed pods; and • fodder or oilseed radish (var. oleifera), which is grown for animal fodder or green manure, apparently no longer grown for oil.

Selected References

Crambe abyssinica The genus Crambe, an old world genus of the tribe Brassiceae, is composed of approximately 30 species. C. abyssinica (n = 45) is an industrial oilseed crop, belonging to the Crambe section Leptocrambe. The seed oil is of considerable economic importance for industrial applications, including use as erucamide (antiblock and slip agent in plastic films), coatings, lubricants (such as metal cutting oils, automatic transmission fluid supplement, and hydraulic fluid), and nylon 1313. C. abyssinica is endemic to the Abyssinian highlands, and the name has been used not only for the wild Ethiopian population but also for the forms cultivated as an oilseed crop. It is derived from C. hispanica L. (n = 30), a widespread endemic of the Mediterranean region. Cultivation of C. abyssinica was apparently initiated in the former Soviet Union and has been inves6

Hanelt, P. 1986. Cruciferae. Pages 272­-­332 in: Verzeichnis landwirtschftlicher und gärtnerischer Kulturpflanzen (ohne Zierpflanzen), Vol. 1. J. Schultze­-­Motel, ed. Springer­-­Verlag, New York.

Harberd, D. J. 1972. A contribution to the cyto­-­taxonomy of Brassica (Cruciferae) and its allies. J. Linn. Soc. London Bot. 65:1­-­23.

Prakash, S., and Hinata, K. 1980. Taxonomy, cytogenetics and origin of crop brassicas: A review. Opera Bot. 55:1­-­57.

Song, K., Osborn, T. C., and Williams, P. H. 1990. Brassica taxonomy based on nuclear restriction fragment length polymorphisms (RFLPs). 3. Genome relationships in Brassica and related genera and the origin of B. oleracea and B. rapa (syn. campestris). Theor.

Appl. Genet. 79:497­-­506.

Specht, C. E., and Diederichsen, A. 2001. Eruca. Pages 1470­-­1472 in: Mansfeld’s Encyclopedia of Agricultural and Horticultural Crops, Vol. 3. P. Hanelt, ed. Springer­-­Verlag, Berlin, Germany.

(Prepared by S. I. Warwick)

Production Management General Considerations Species and Cultivar Selection

Selection of brassica species and cultivars should be made with regard to physiological and environmental limitations and with knowledge of potential disease and insect problems. Exposure of some brassica species to low temperatures can cause premature bolting, particularly in early­-­maturing cultivars.

Since flower stalk formation is genetically controlled, selection of cold­-­tolerant cultivars becomes important in some growing regions. State or regional lists show which cultivars are best adapted to local conditions. Cultivar selection is also determined by market demand as related to morphological characteristics.

Examples include round­-­versus conical­-­headed cabbage, curly­-­ versus flat­-­leaf kale, or physical size of some ethnic vegetables.

Fertilization

Brassicas are generally cool­-­weather crops that prefer deep, well­-­drained, fertile, friable, sandy or silt loam soils. Optimum soil pH is 6.0–6.5 for most species, but some are more tolerant to greater soil acidity than are others. Application of fertilizers should rely on regional production recommendations for specific crop species and current soil test results. Most nutrients are applied preplant. For both transplanted and precision­-­seeded crops, such as cabbage, broccoli, cauliflower, Brussels sprouts, and some Asian vegetable brassicas, banding some of the fertilizer 5–10 cm from the young roots aids in establishment and early growth (Fig. 2). For longer­-­season cultivars with later maturity, from one­-­third to one­-­half of the total nitrogen requirementissupplementedpostplantduringtheearlyvegetative phase of growth. Excess nitrogen in broccoli and cauliflower can predispose plants to hollow stems and delay head or curd formation. Most brassica crops have a significant boron requirement, ranging from 1 to 4 kg/ha. Additional boron, sulfur, and other micronutrients may be needed in sandy soils or soils naturally low in these elements. Many brassica crops, particularly those grown as vegetables, are sensitive to molybdenum deficiency.

The management of soil pH can be critical for these crops since molybdenum availability, like that of phosphorus, is reduced below a soil pH of 5.5. Application of excess micronutrients, such as boron, can cause plant injury and reduce yields.

Fig. 2. Side banding of ammonium nitrate. (Courtesy V. I. Shattuck)

Direct Seeding

The planting method used depends on the crop to be grown, the growing region, and the target market. Most brassica crops grown for seeds, forage, or as cover crops are direct­-­seeded with a seed drill. In some areas, vegetable brassicas, such as cabbage, broccoli, cauliflower, greens, and many of the Asian vegetables, are direct­-­seeded with precision seeders, which can greatly improve plant spacing, reduce the need for costly hand thinning, and reduce seed costs (Fig. 3). Adequate soil preparation is essential for precision seeding. To avoid poor stands, seeding should take place when soil moisture and temperature conditions favor rapid germination. Planting certified seeds helps reduce the incidence of seedborne disease problems and enhances germination and early plant vigor, critical for uniform maturity and efficient harvest. Often, brassica vegetable seeds are routinely treated with fungicides or with hot­-­water treatment to reduce the level of or eliminate Xanthomonas campestris pv. campestris (black rot) and other pathogens. Seed “priming” is a controlled hydration treatment followed by drying that is sometimes used to improve germination rate. However, priming reduces the storage life of seeds, particularly under adverse storage conditions.

Transplanting

For the early production of cabbage, cauliflower, and broccoli or for production in areas where plant establishment by direct seeding is a problem because of temperature or moisture stress, transplants may be used. Brussels sprouts are usually transplanted to shorten the growing season and to improve uniformity at harvest. Plants can be produced in specially prepared seedbeds in the field and transplanted bare rooted, or they can be grown in plug or cell trays in a greenhouse and transplanted with a root ball (Fig. 4).

Seeding depth should be 1.5–3.0 cm, depending on soil temperature and available moisture. The optimum distance between seeds is 1–2 cm. Once seeds germinate, plants in the juvenile phase tolerate short­-­term exposure to temperatures as low as 0 to –5°C with little adverse effect. However, if plants have more than four to five true leaves and the stem diameter is greater than 8 mm, they may become vernalized and subject to premature bolting. Germination is poor at temperatures exceeding 30°C, the ceiling temperature for many brassica

Fig. 3. Field of mechanically planted red cabbage. (Courtesy V. I. Shattuck)

7

s­ pecies. The order of tolerance to high temperatures for the major vegetable members of B. oleracea is cabbage (most tolerant), broccoli, cauliflower, and Brussels sprouts (least tolerant).

Conditions that favor rapid growth, i.e., high levels of nitrogen, excess moisture, or both, resulting in transplants with heavy foliage and few roots, should be avoided. Field­-­grown plants are ready to transplant 5–8 weeks after sowing, depending on soil temperature and weather conditions.

Alternatively, transplants may be produced in trays or flats in which seeds are planted in individual cells filled with a soilless media. Although the initial cost of tray­-­grown plants is higher, the survival rate of transplants in the field is improved because the root ball remains intact. Float­-­bed systems, in which flats are seeded and floated in water or nutrient solution, may also be used. At 16–18°C, tray­-­grown plants should be ready for transplanting in 4–7 weeks. Optimum transplant height is 15–20 cm, and oversized plants can be more difficult to transplant mechanically and establish than can younger, smaller plants.

Transplants should not be pruned or trimmed, since this may remove the apical meristem and stimulate shoot growth at the expense of root growth. Cutting transplants also promotes the spread of plant pathogens, particularly bacterial pathogens.

Transplants are hardened before field planting to reduce shock. Hardening is generally accomplished by withholding water, reducing nitrogen levels, and exposing plants to low temperatures and direct sunlight, either alone or in combination.

This results in plants with short, sturdy stems, well­-­developed root systems, and greater dry weight. Transplanting is best conducted under cool, moist conditions that limit water loss. A liquid fertilizer solution, often in combination with a pesticide, is usually applied at transplanting.

Pest Management

Control of plant pathogens, insect pests, and weeds should be anintegratedcomponentofproductionmanagement.Development of an integrated pest management system should rely on regional recommendations and be in accordance with labeling restrictions for chemical control measures.

Brassica crops should be grown in a 3­-­ to 5­-­year rotation with nonbrassica crops to reduce the risk of soilborne diseases.

Longer rotations may be needed when soilborne diseases, such as black leg (Leptosphaeria maculans) or clubroot (Plasmodio­ phora brassicae), are present. Controlling weeds is essential to avoid yield reduction from competition and to increase air circulation in the planting, thereby reducing the incidence of diseases. Introduction of plant pathogens, root­-­dwelling insects, nematodes, and weed seeds from soil­-­laden farm equipment, transplants, and irrigation water should be avoided. When possible, certified seeds, free of disease and weeds, should be used.

The insect pest management program should keep pest populations below damaging levels, i.e., the economic threshold, while protecting natural enemies and competitors. Registered pesticides, i.e., herbicides, fungicides, and insecticides, should be selected based on target species, efficacy, and harvest intervals and used with appropriate rates and application methods.

Irrigation

Irrigation systems are often needed to ensure uninterrupted plant growth and to promote uniform maturity of many vegetable brassica species. Damage by nematodes, root diseases, insects,andweedcompetitionisalsomoreapparentonmoisture­­stressed crops. For heading brassica crops, such as cabbage, broccoli, and cauliflower, it is particularly important to avoid water stress during head development. In many areas, the final irrigation is scheduled a few days to 1 week before harvest to ensure that plants are fully hydrated. Sprinkler irrigation should be applied no faster than the rate of water infiltration to avoid surface puddling or runoff. Crop moisture requirements change as the crop develops. Excessive moisture or poor drainage may contribute to the development of black rot (Xanthomonas campestris pv. campestris) and Phytophthora root rot. For areas where water quality is a problem, i.e., high salt concentration, keeping the soil surface wet minimizes the upward movement and accumulation of salts near the surface.

If sufficient water is available, salts can sometimes be leached below the rooting zone of the plants.

Harvest and Storage

Fig. 4. Cauliflower transplants grown in cell trays. (Courtesy V. I. Shattuck)

For vegetable crops grown for fresh market, optimizing crop yields and maximizing product quality are high priorities. Harvesting should be completed in a timely manner when the crop has reached commercial or physiological maturity and, if well planned, to coincide with optimal market prices. Precooling to remove field heat and storing at optimum temperature and relative humidity prolongs shelf life and reduces postharvest losses (Fig. 5). Several precooling techniques may be used, such as room cooling, forced­-­air cooling, hydrocooling, vacuum cooling, and slurry icing. Equally important is the ventilation and regulation of atmospheric levels of oxygen and carbon dioxide during storage.

Vegetable Crops Cabbage Fig. 5. Wax­-­impregnated carton containing slurry­-­iced broccoli heads. (Courtesy V. I. Shattuck)

8

Cabbage (Brassica oleracea var. capitata) is grown for both fresh market and processing. Many of the current cabbage cultivars are hybrids and some are resistant to one or more dis-

eases and insect pests. Although production of late­-­maturing cabbage has declined, it is still worth more than $80 million in New York State alone and supplies much of the cabbage for coleslaw in the eastern United States.

Cultivar Selection

The most commonly grown cultivars are green, but there are red­-­leafed and savoy or crinkled­-­leafed cultivars as well.

There are also several distinct types of cabbage heads, which vary in both size and shape. Growing red and savoy types along with fresh market green cabbage cultivars may improve sales in some areas. Cultivar selection should be based on several factors, which include adaptability, but in particular, the preferences and timing of the target market should be considered.

Examples of cultivars types with various characteristics are listed below.

• Wakefield­—­small, pyramid­-­shaped head; early maturing; and cold tolerant and resistant to bolting.

• Copenhagen­—­round head with few wrapper leaves and small core; early maturing; and susceptible to bolting.

• Flat Dutch­—­large, flat, and solid head with numerous wrapper leaves; and maturity varies from early to ­midseason.

• Danish­—­moderate plant and head size and solid heads with few wrapper leaves, covered with fairly heavy bloom; late maturing for fall production; and keeps well for fresh market, processing, and storage.

• Alpha­—­small head; and early maturing (of little commercial importance).

• Volga­—­large head with large, thick, steel blue­-­colored leaves that are solid on top, but open below; and late maturing (not widely grown).

Fertilization

Cabbage grows well in both mineral and organic soils with adequate moisture and fertility. Usually, little additional nitrogen is needed for cabbage grown in organic soils. Excessive nitrogen can promote secondary growth and split heads, as well as decrease storage life. When combined with high temperatures, excess nitrogen can also promote such rapid growth that plants of some cultivars show symptoms of tipburn. In contrast, nitrogen deficiency delays maturity, depresses yields, and causes objectionable flavor. Like most brassica species, cabbage has a high requirement for boron and molybdenum.

Planting

Cabbage may be planted on flat ground or in raised beds.

Plant spacing can control head size. Smaller heads are preferred for fresh market. In­-­row spacing between 25 and 40 cm is common for early­-­maturing cultivars, while 40–70 cm are needed for late­-­maturing cultivars. Closer plant spacing reduces the risk of cracked heads and tipburn, but crop maturity may be delayed. Wide plant spacing and high fertilization rates increase the risk of problems with auxiliary heads or small heads in direct­-­seeded cabbage but rarely in transplanted crops.

Well­-­hardened transplants withstand –7°C for short periods.

Generally, young plants are more tolerant of low and high temperatures than are plants approaching maturity. The optimum mean temperature for growth and head quality is 15–18°C, with approximate minimum and maximum temperatures of 4 and 24°C, respectively.

sive moisture may delay maturity and reduce yields, especially in early­-­maturing cultivars.

Harvest

Most cabbage for fresh market is harvested by hand. Hybrid cultivars usually require only one harvest because of their greater uniformity. As with all leafy vegetables, removing field heat is critical to maintain quality, and hydrocooling or forced­­air cooling is most commonly used. Storage life varies dramatically based on the type of cabbage and the growing conditions in the field. Most fresh market cultivars can be stored from 30 to 60 days at 0°C and a relative humidity above 90%. Cabbage cultivars especially developed for long­-­term storage may be held for extended periods (6 months or more). Cabbage should not be stored with fruits that produce appreciable amounts of ethylene, since this causes yellowing of the wrapper leaves, detachment of the core and leaves, or both.

Cauliflower, Broccoli, and Brussels Sprouts Cultivar Selection

Both open­-­pollinated and hybrid cultivars of cauliflower are available. The cultivars are grouped into early­-­maturing ‘Snowball’ types and late­-­maturing winter types. Cauliflower with green curds is marketed as “broccoflower” or “broccoli­­flower”, but it is not a hybrid between broccoli and cauliflower.

For the white­- ­curd fresh market, cauliflower heads should be uniform in shape, with clean, unbruised, pure white to light cream curds. In the processing market, there is greater acceptance of off­-­color curds, which are whitened during processing.

When exposed to sunlight, curds turn from white to an off­­cream or brownish color. Certain cultivars, especially early­­maturing types with poor wrapper leaf development, are more prone to off­-­color curd development. The earliest developed curds of early­-­maturing cultivars can be protected from direct sunlight by tying the wrapper leaves over the curd, a technique called blanching (Fig. 6). Some cultivars of the ‘Snowball’ type have large leaves that protect the curd naturally (self­-­blanching) and do not usually require tying. The winter types of cauliflower require vernalization ranging from a few days to several weeks, depending on the cultivar.

Most broccoli cultivars are hybrids (Fig. 7). Cultivar selection should be based on days to head maturity and head quality, since tight heads with fine beading, i.e., small floret size, is preferred for fresh market. Broccoli cultivars that produce green or purple bud clusters are available. Most of the early­-­and medium­-­maturing cultivars are the sprouting

Irrigation

Generally, cabbage requires from 2.5 to 3.8 cm of water per week for uninterrupted growth. Yield reduction may occur when the soil moisture level remains below 50% of field capacity for extended periods, particularly in sandy soils. Drought during the juvenile stage (three to four true leaves, with a stem diameter of less than 8 mm) may adversely affect plant development and uniformity of maturity. On the other hand, exces-

Fig. 6. Blanching of cauliflower curds. (Courtesy V. I. Shattuck)

9

Fig. 7. Production of hybrid broccoli seeds. (Courtesy R. H. Morrison)

Fig. 8. Hand harvest of broccoli with harvesting aid in Ontario, Canada. (Courtesy V. I. Shattuck)

­ roccoli type, which require high temperatures (27°C) to prob duce a head. Late­-­maturing cultivars either of the sprouting or heading broccoli type, which require vernalization, are grown in areas where they can overwinter and be harvested in late winter or early spring.

Brussels sprouts are generally produced with hybrid cultivars, since these have improved uniformity of harvest. Cultivars of Brussels sprouts are often grouped as early, main season, or late according to maturity, which ranges from 85 to 125 days from transplanting. Cultivars with smaller sprouts are often grown for freezing.

Cauliflower curd development has a narrow optimum temperature range of 14 to 20°C and is sensitive to extreme temperatures. Exposure to near­-­freezing temperatures damages cauliflower shoot apices and prevents curd development. High temperatures may cause reversion to vegetative growth or the development of small leaves and bracts within the curd. Less­­than­-­ideal environmental conditions can also result in the loss of compactness and the development of “ricy” curds. Some newer broccoli and Indian cauliflower cultivars are quite heat tolerant. Brussels sprouts require a long growing season and transplanting is normally required. Plants are relatively hardy and withstand temperatures as low as –5°C.

Fertilization

Nitrogen deficiency and exposure of young broccoli or cauliflower plants to low temperatures or environmental stress, ­either alone or in combination, can lead to premature heading, which is also called buttoning. Boron stress can cause stem cracking and browning of individual florets in broccoli, and in cauliflower, water­-­soaked areas on stems and branches of the curd may develop. Broccoli and cauliflower are also sensitive to molybdenum deficiency, which causes whiptail. Cauliflower is sensitive to magnesium and manganese deficiencies, which are expressed as interveinal chlorosis on the older and younger leaves, respectively.

Deep, loamy, well­-­drained soils are needed for the production of cauliflower and Brussels sprouts. Broccoli can be grown on a wider variety of soils, although better yields may be obtained on lighter soils, such as sand or silt loams, rather than on clay or clay loams. Long­-­season cultivars of cauliflower and Brussels sprouts should receive supplemental nitrogen. For processed cauliflower, higher­-­than­-­normal nitrogen rates are often used to maximize head size and curd density.

Planting

In California and most of the western United States, broccoli and cauliflower are direct­-­seeded on raised beds and thinned while the plants are small. For processing, bare­-­root transplants are often used. In the eastern United States, broccoli may either be direct­-­seeded or transplanted; however, direct seeding of cauliflower is not recommended, since exposure to environmental stress during early plant development often results in buttoning. Cauliflower is transplanted into raised beds with 60–90 cm between plants and 90­-­cm row spacing, although spacing may depend on the cultivar. Spacing of broccoli plants varies from 20 to 60 cm with 50–90 cm between rows, and multiple rows on raised beds are widely used. The spacing for broccoli depends on the requirements of the market and the incidence of local disease problems. Brussels sprouts are spaced the same as cauliflower, but for once­-­over harvest, plant spacing may be as narrow as 30–40 cm with 70­- ­cm row spacing, which delays maturity.

10

Harvest

Fresh market cauliflower and broccoli are harvested by hand (Fig. 8). Because of its high respiration rate, broccoli must be cooled immediately after harvest to maintain head quality and color (Fig. 5). Broccoli should be stored at 0°C and 95–100% relative humidity. Shelf life is relatively short (7–10 days) and may be significantly shorter if delays in cooling or fluctuations in storage temperature occur. Careful handling is needed for both broccoli and cauliflower to minimize bruising, which reduces shelf life. Cauliflower may be stored at 0°C and 95–98% relative humidity for 3–4 weeks. However, freeze damage can occur at –1°C, resulting in discoloration and softening of the curd.

Harvest commences when Brussels sprouts are firm, well developed, and approximately 2.5–5 cm in diameter. Brussels sprouts for both fresh market and processing are often harvested mechanically. For once­-­over mechanical harvest, applying a tip inhibitor or physically removing the growing point when the lower sprouts are about 1.3 cm in diameter stimulates uniform sprout development. For the fresh market, several successive hand harvests may be made, starting at the base of the plant.

As the lower sprouts are removed, those further up the stalk continue to develop. Lower temperatures or even a slight frost increases the compactness and flavor of the sprouts. Brussels sprouts should be cooled after harvest and stored at 0°C and 95–100% relative humidity. Like cabbage and broccoli, Brussels sprouts should not be stored with fruits that generate ­ethylene.

Kohlrabi Kohlrabi (Brassica oleracea var. gongylodes) is a biennial plant grown as an annual vegetable for its enlarged stem. Kohlrabi is more widely grown in Europe than in the United States.

Cultivar selection in the United States is limited, with ‘White Vienna’ being the most common. Green and purple cultivars are produced for niche markets.

Rich, loamy soils or generous applications of organic matter aid the production of tender stems. Fertilization and cultivation requirements are similar to those needed for cauliflower. Excess nitrogen and water can cause stem cracking in some cultivars. The best quality stems are obtained by encouraging rapid and continuous growth. The optimum temperature for kohlrabi growth and development is 16–21°C. Exposure to temperatures below 7°C can result in vernalization and bolting, while high temperatures retard growth and promote tough, stringy tissue.

Although kohlrabi can be transplanted, it is more typically direct­-­seeded and thinned to 10–16 cm between plants, or it may be precision­-­seeded to a final stand. Successive plantings every 2–3 weeks ensure a continuous supply throughout the season.

Kohlrabi should be harvested when the swollen stem is 5–8 cm in diameter and before it is tough and woody, normally 55–65 days after seeding. The root is cut off, and stems may be sold individually, bunched by the top, or bagged prior to sale. The recommended storage temperature is 0°C, but quality can only be maintained a few days when the tops are left on. Removal of the tops and storage at 0°C can extend storage life up to 4 weeks.

Greens Greens are a diverse group of nonheading brassica species, including mustard (Brassica juncea), turnip (B. rapa subsp. rapa), and kale and collards (B. oleracea var. viridis).

Greens are grown for their leaves, which are either used fresh or cooked. In parts of Canada, young rutabaga leaves (B. napus subsp. rapifera) are also used as greens. The greens are cool­­weather crops that are planted in the summer or in the fall in the southern United States. Certain cultivars of turnips have been developed for use only as greens and do not form enlarged roots.

Many cultivars are dual purpose and both the roots and leaves may be used as vegetables. Both the Siberian (smooth­-­leaf type) and Scotch (curly­-­leaf type) kales are widely grown. Currently, most kale grown in the United States is of the curly type, of which cultivars with either blue or green leaves are available.

Production management is the same for all greens and similar to that of cabbage. Greens are best grown on a well­-­drained, sandy loam soil. In some areas, double­-­or triple­-­seeded, tray­­growntransplantsareplantedinplasticmulchfortheproduction of bunch­-­harvested collards. Although some kale and collards are transplanted, most are direct­-­seeded and are thinned only if necessary. Adequate fertility is needed to maintain vegetative growth, but excessive nitrogen may reduce cold tolerance and storage quality and increase the severity and incidence of tipburn. The growing season may be as short as 40–50 days for kale and mustard but is usually 75–90 days for collards. Collards, mustard, and kale, in particular, tolerate temperatures as low as –9°C, if not preceded by warm weather. These crops can be harvested after frost until they are eventually killed by subfreezing temperatures. Leaves of collards and kale are harvested sequentially as they reach full size but are still tender.

Collards may also be harvested by cutting the rosette at ground level when plants are 15–30 cm tall. Leafy greens are highly perishable, and field heat should be removed quickly by icing, hydrocooling, or vacuum cooling. The optimum storage temperature is 0°C with 95–100% relative humidity, but storage life is usually only 1–2 weeks.

Asian Vegetable Brassica Crops The genus Brassica includes many vegetables that are widely grown in Southeast Asia and, to a lesser extent, in other parts

of the world. A discussion of these vegetables is complicated by the many common names derived from different languages.

However, Asian vegetables may be grouped as Chinese cabbage (B. rapa), Asian mustard greens (B. juncea), and Chinese broccoli or Chinese kale (B. oleracea).

Asian Mustard Greens Asian mustard greens (Brassica juncea) comprise a large and diverse group that includes Chinese green mustard or gai choi, mizuna, and various potherb mustards. Mustard leaves may be eaten raw in mixed salads, and the gai choi cultivars can be prepared like spinach.

Asian mustard greens are grown as annuals and should be produced at the same time of year as other cool­-­season greens.

The requirements for climate, soil fertility, irrigation, and postharvest handling are similar to those described for non­heading types of Chinese cabbage. Most Asian mustard greens are direct­-­seeded into flat or raised beds. Leaves may be harvested when plants are at least 3 weeks old. If plants are to be harvested when 10–15 cm tall, the seeds can be broadcast. However, larger plants have greater pungency and may be harvested 45–70 days after planting, when they are 15–20 cm tall.

Mizuna (B. rapa subsp. nipposinica) is widely grown in Japan and is gaining popularity in North America because it withstands light frost, is relatively heat tolerant, and resists bolting better than Chinese cabbage. Plants are 30–46 cm tall with yellow­-­green, smooth, and slightly pubescent leaves that have a deeply notched, narrow, feathery shape.

Chinese Broccoli (Chinese Kale) Chinese broccoli (Brassica oleracea var. alboglabra) is also known as gai­-­lon (or gai­-­lohn), kailan, or Chinese kale. The Chinese broccoli plant resembles sprouting broccoli, except that the leaves are a bit broader, the stems are longer, and the head is much smaller. Flowers form first in diminutive heads and then elongate rapidly into stalks bearing yellow or white flowers similar to those of broccoli raab. Chinese broccoli is grown as an annual like other types of broccoli.

Phosphorus and potassium requirements of Chinese broccoli are similar to those of Chinese cabbage, but the nitrogen requirement is higher. Seeds are planted 1.3 cm deep, 2.5–5 cm apart, and in rows spaced 46 cm apart. Plants are generally thinned to 15 cm when established. The crop needs at least 2.5 cm of water per week and matures about 55–60 days after seeding. Heads, including about 20 cm of the stalk and a few leaves, are harvested just before the flowers open. The timing of harvest is critical since the plants grow rapidly and can only be marketed successfully at the correct stage of development.

However, each plant can be harvested several times. Harvested heads should be stored at 0°C and 95–100% relative humidity.

Under optimal conditions, storage life is 10–14 days.

Chinese Cabbage Chinese cabbage is the most widely grown Asian vegetable in North America and includes heading types (Brassica rapa subsp. pekinensis) and nonheading types (B. rapa subsp. chinensis). Heading types may be further classified according to head shape, such as open, erect, cylindrical, ovoid, or barrel.

Nonheading types can be classified according to growth habit, such as upright growth with prominent petioles, rosette habit with short petioles, or flowering heads on a fleshy stem.

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clods, is needed to facilitate precision seeding and uniform root development.Stonesmayincreasetheincidenceofbranchedand crooked roots, especially in radishes. Stones in the soil also increase the occurrence of bruising of root crops during harvest.

Horseradish

Fig. 9. Hand harvest of Chinese cabbage near Beijing, China.

(Courtesy V. I. Shattuck)

Chinese cabbage is generally annual, sometimes biennial, and does best under daily average temperatures between 13 and 21°C. Low temperatures between 4 and 10°C and long days (15–16 h) for 4–5 weeks induce flowering in cultivars susceptible to bolting.

The crop requires a rich, well­-­drained soil with a high water­­holding capacity. Usually about one­-­half of the nitrogen should be applied at seeding and the other half shortly after thinning.

Excess nitrogen results in fewer marketable heads and may increase susceptibility to rot during shipping and storage. All phosphorus and potassium should be applied before or at planting. Phosphorus deficiency may occur in pak choi during cool and wet weather, particularly in heavier soils. Excess potassium does not improve yields. Additional boron may be needed in some soils.

Chinese cabbage can be grown on either flat or raised beds, depending on soil type and climate. Spring crops are generally transplanted, while fall crops are most often direct­-­seeded at a depth of 1.3 cm and thinned to the desired stand. The plant spacing for Chinese cabbage ranges from 31 to 46 cm for heading types and from 20 to 31 cm for nonheading types. Cultivation shouldbeshallowtoavoidrootdamage.Irrigationneedsdepend on soil type and weather conditions, but generally a total of 2.5 cm per week from irrigation, rain, or both is ­necessary.

Nonheading leafy types reach marketable size in 35–60 days; the entire plant may be harvested or individual leaves removed for multiple harvests. Heading types are hand­-­harvested from 65 to 100 days after seeding by cutting the entire head at ground level and removing the outer leaves (Fig. 9). Chinese cabbage may be stored for several weeks at 0°C and 95% relative humidity. Storage life is severely shortened at high ­temperatures.

Root Crops Annual brassica root crops include turnip (Brassica rapa subsp. rapa), rutabaga (B. napus subsp. rapifera), and radish (Raphanus sativus). Horseradish (Armoracia rusticana P. G. Gaertn., B. Mey., & Scherb.) is an herbaceous perennial. The annual root crops prefer lower temperatures for growth and development. Long days and higher temperatures promote seed stalk elongation. An adequate supply of nitrogen, phosphorus, and potassium is required to promote rapid, continuous growth of annual root crops. In some soils, boron may be required, otherwise brown heart may develop in the root tissue. For all direct­­seeded brassica root crops, a fine seedbed, free of stones and 12

Horseradish (Armoracia rusticana P. G. Gaertn., B. Mey., & Scherb.) is an herbaceous perennial grown for its pungent roots, which are grated and eaten fresh or added as an ingredient to other food products, such as seafood sauce. Horseradish has a wide climatic adaptation. It is grown primarily as an annual but may also be grown in a perennial production system. Horseradish is a long­-­season crop, and higher temperatures are needed during the vegetative phase, followed by lower temperatures to enhance root development. In the annual system, the crop is harvested after the first killing frost. In the perennial system, upright, thickened shoots arising from a “mother” plant are harvested every other year, with the original plant left in the field for regeneration. Perennial fields may stay in production for 10–20 years.

Horseradish is propagated vegetatively by root cuttings (sets) that are 1.25–3.5 cm in diameter and 20–40 cm long. In the annual system, this set enlarges and becomes the primary marketable product at season’s end, along with the many secondary roots forming along the medial and distal regions. There appears to be no vernalization requirement, and sets can be planted anytime after separation from the main plant.

Approximately 2,900–3,500 root cuttings per hectare are needed to establish a new crop. Generally, rows are spaced 0.9–1.2 m apart, with 45–60 cm between plants. Horseradish grows best on rich, moist, deep, friable, loam or sandy loam soils rich in organic matter. Good drainage is needed for the production of quality roots. Horseradish grown in soils with hard subsoils frequently produce highly branched roots of poor quality, and the set sizing is poor. Horseradish has a high potassium requirement, moderate phosphorus needs, and low to moderate nitrogen requirements. Irrigation during dry periods, particularly in late summer to fall, can improve marketable yields.

Radish Radish (Raphanus sativus) types can be grouped by root maturity. The early­-­spring or garden types mature in 25–40 days and have round or long roots that are usually white, red, or bicolored. The late­-­maturing winter types include Spanish radish, daikon, and Chinese radish. These take much longer to develop and have larger roots weighing up to 20 kg, a wide range of root colors, and a much longer storage life. Round­-­rooted, spring­­type cultivars are more likely to bolt than are long­-­rooted cultivars, which are more resistant to high temperatures.

Radish is more tolerant of a low soil pH of 5.5–6.0 than are broccoli, cabbage, cauliflower, or Chinese cabbage, but it grows better at pH 6.0–6.8. Light, friable soil is considered best, although radish can be grown in most soil types. Spring radish is usually planted at quite dense populations on either flat or raised beds, while winter radish requires more space and may need thinning to 5–10 cm within rows. There are successive seedings of spring radish, since the crop matures in only 3–5 weeks. Winter radish requires twice as long to mature. Spring radish requires high levels of nitrogen for sustained growth and optimum root quality. Roots can be stored at 0°C and 95–100% relative humidity for 3–4 weeks. Winter radish needs to be handled carefully to avoid bruising. At 0°C storage temperature, winter radish should keep for 3–4 months.

Fig. 10. Mechanical harvest of rutabaga. (Courtesy V. I. Shattuck)

Turnip and Rutabaga Cultivars of rutabaga (Brassica napus subsp. rapifera), also known as swede, and turnip (B. rapa subsp. rapa) are available with white or yellow roots. Roots of most rutabaga cultivars are yellow and larger than those of the white­-­fleshed turnip cultivars. The color on the root shoulder may be bronze, green, or purple. Turnip and rutabaga are also grown as forage crops or winter cover crops in some areas. Since rutabaga contains substantial amounts of S­-­methylcysteine sulfide, precautions should be taken to prevent animal health problems.

Turnip and rutabaga can be grown in all types of soils, although yields are better in deep, rich, well­-­drained loams. Both vegetables are grown as row crops, but rutabaga requires more space than does turnip. Turnip and rutabaga are sensitive to boron deficiency. Roots of turnip have the best quality when they are 5–7.5 cm in diameter, usually reached in about 60 days.

The preferred diameter for fresh market rutabaga ranges from 6 to 15 cm, which requires 4–5 weeks of additional growth.

Turnip and rutabaga may either be pulled by hand or machine­­harvested (Fig. 10). When marketed directly after harvest, rutabaga roots are sometimes waxed to reduce the likelihood of shriveling and weight loss. Both crops require a temperature near freezing and high relative humidity during storage, and they keep for 2–6 months.

Oilseed Crops Oilseed Rape (Canola, Rapeseed) The main oilseed Brassica spp. are oilseed rape (B. napus subsp. oleifera), oilseed turnip rape (B. rapa subsp. ­oleifera), and oilseed mustard (B. juncea). Oilseed rape is widely adapted, particularly to cool­-­season areas, while oilseed mustard is grown predominantly in India and Pakistan.

Cultivars of oilseed Brassica spp. in which the processed oil contains less than 2% erucic acid and the residual meal contains less than 3 mg of glucosinolates per gram (or <10 mol/g of seeds) are named “canola”. Erucic acid is a long­-­chain fatty acid that has been related to heart disease in experiments with rats. Glucosinolates break down to yield products that are toxic to animals, especially monogastric animals, such as hogs and poultry. “Canola” was registered in 1979 by the Canola Council of Canada to describe a high­-­quality vegetable oil with low levels of saturated fats, suitable for human consumption.

Oilseed rape grows best on medium­-­textured, well­-­drained soils. The crop is tolerant of soil pH as low as 5.5 and of saline

Fig. 11. Oilseed rape in windrows. (Courtesy S. R. Rimmer)

conditions. Winter cultivars are fairly heavy users of nitrogen.

Frequently, some nitrogen is incorporated before planting (preplant), with the balance top­-­dressed in early spring. Excessive nitrogen application in the fall can promote vigorous growth that tends to make young plants less winter hardy. Oilseed rape has a relatively low phosphorus requirement, similar to that of wheat. In contrast, proper potassium levels help increase oil content. Therefore, soil potassium content should be raised to the recommended level if deficient. All required phosphorus and potassium are applied prior to planting. Additional sulfur may not be required if the residual sulfur in the top 30 cm exceeds 4 ppm. Excessive sulfur levels can increase the glucosinolate content of the meal and lower the quality. Rapeseed is sensitive to boron deficiency. All required sulfur and boron is applied preplant.

Oilseed rape can be seeded in either the fall or spring, depending on the cultivar. Winter types are grown where snow covers the ground or mild winters are common. Timing of fall planting is important for survival. Six true leaves and good root reserves are optimal at the start of winter. It should be seeded at 5.0–5.5 kg/ha when drilled and 8–9 kg/ha if broadcast to obtain stands of around 65–85 plants per square meter. Maintaining a crop with a reduction in plant population density of up to 65% after overwintering is more profitable than reseeding. Spring planting should begin as soon as the soil is dry and weather permits. The crop germinates at a soil temperature of 5°C, but the optimum is 10°C. Oilseed rape requires approximately 41–46 cm of water through the growing season, with one­-­half used during the flowering and pod­-­filling stages. Weed control in the field is important since seeds of many weed species, particularly other crucifers, are difficult to separate during seed cleaning, and these may reduce oil quality.

Oilseed rape is more susceptible to shattering than is oilseed turnip rape, and it is often harvested by windrowing (swathing) first and combining later (Fig. 11). Windrowing is done when about one­-­third of the seeds in the pods have turned dark brown. The windrow is placed on 15­-­to 25­-­cm­-­tall stubble to dry for 7–10 days. When the seed moisture content has reached approximately 9%, the crop can be combined and stored. Oilseed rape can also be harvested by direct combining when seed moisture content reaches 8–10% or at a higher moisture content if the seeds can be dried artificially. Drying with heat should not exceed 40°C to avoid changes in oil composition and seed damage. The seed moisture content should not be reduced to below 6% since the seeds become brittle and subject to ­damage.

Culinary or Condiment Mustards Yellow mustard (Sinapis alba) and brown and Oriental mustards (Brassica juncea) are grown for their seeds, which are 13

processed into paste or powder for condiments. Other members of the species B. juncea are grown as an oilseed crop, greens, and stem vegetables. Yellow mustard constitutes about 90% of the crop in North America, while brown and Oriental mustards are grown on a small scale. In Europe, yellow mustard is also known as white mustard.

Mustard is an annual that is planted in the spring. It emerges rapidly but grows slowly. It can be grown on various soil types with good drainage but is best adapted to fertile, well­-­drained, loamy soils. Growth will be stunted if the crop is grown on waterlogged soils. Drought­-­prone sand and sandy loam soils should also be avoided. B. juncea, however, is more heat and drought tolerant than is S. alba. Soils prone to crusting prior to emergence can cause problems.

The seedbed should be firm, fairly level, and free of weeds and crop residue. Shallow tillage, just deep enough to kill weeds, is preferable to maintain soil moisture close to the surface and leave a firm seedbed. Minimum tillage systems have also been successful. If necessary, the seedbed should be packed before planting.

Fertilization requirements are similar to those of oilseed rape and are applied preplant, but soil tests should be used to determine actual nutrient needs. Mustard responds well to nitrogen additions and to sulfur on some soils. In soils deficient in boron (less than 0.5 ppm), 0.6–1.1 kg/ha should be applied. Soils with a pH near 7.0 are desirable; nevertheless, mustards tolerate an alkaline pH and slightly saline soils.

Yellow mustard is seeded at 9–16 kg/ha; the higher rates are appropriate in heavy, fertile soils or in those where emergence is difficult. Brown and Oriental mustards have smaller seeds and should be planted at 6–8 kg/ha and no deeper than 1.3–2.5 cm. If very dry soil conditions exists, seeding depth should be increased to 3.5 cm.

Seedlings are usually somewhat tolerant to mild frosts after emergence, but severe frosts can destroy the crop. The taproots grow 1.5 m into the soil under dry conditions, allowing for efficient use of stored soil moisture. Plants cover the ground in 4–5 weeks under favorable moisture and temperature conditions.

Flower buds are visible about 5 weeks after emergence. Yellow flowers begin to appear 7–10 days later and continue blooming for a long period if the water supply is adequate. If moisture stress occurs, yields are reduced.

Cultivars of yellow mustard usually mature in 80–85 days, whereas brown and Oriental types require 90–95 days. Yellow mustard does not shatter readily, and if it is free from green weeds, the crop can be combined directly at 12–13% moisture content followed by artificially drying the seeds. If the crop is weedy or uneven in maturity, it should be windrowed when 60–70% of the seeds have turned yellow. Plants are cut just beneath the lowest seed pods, so the top settles into the stubble and reduces the effect of high winds. Brown and Oriental cultivars shatter more readily when ripe and should be windrowed

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when the overall field color has changed from green to yellow­­brown, when pods from the middle of the racemes have 75% yellow or brown seeds. The remaining green seeds mature in the windrow before combining. The harvested seeds should be handled carefully since they crack easily. Air temperatures for drying should not exceed 50°C, and the seed temperature should stay below 35°C during the drying process. Mustard seeds are optimally stored at a moisture content of 10%.

Selected References Canola Council of Canada. 2004. Growing Canola. http://www. canola­- ­council.org/portal.html.

Flint, M. L., ed. 1985. Integrated Pest Management for Cole Crops and Lettuce. Publ. 3307. University of California, Statewide Integrated Pest Management Project, Division of Agriculture and Natural Resources, Oakland, CA. Hemingway, J. S. 1995. The mustard species: Condiment and food ingredient use and potential as oilseed crops. Pages 373­-­383 in: Brassica Oilseeds­—­Production and Utilization. D. Kimber and D. I. McGregor, eds. CAB International, Wallingford, Oxon, U.K. McClung, C. A., and Schales, F. D. 1982. Commercial Production of Horseradish. Publ. HE 127­-­82. University of Maryland, Cooperative Extension Service, Horticulture Production Series, College Park, MD. Myers, C. 1991. Specialty and Minor Crops Handbook. Publ. 3346.

University of California, Small Farm Center, Davis, CA. Nieuwhof, M. 1969. Cole Crops: Botany, Cultivation, and Utilization.

World Crops Series. Leonard Hill, London.

Oplinger, E. S., Hartman, L. L., Gritton, E. T., Doll, J. D., and ­Kelling, K. A. 1989. Canola (Rapeseed). Pages 1­-­7 in: Alternative Field Crops Manual. University of Wisconsin Cooperative Extension, Madison, WI. Oplinger, E. S., Oelke, E. A., Putnam, D. H., Kelling, K. A., ­Kaminski, A. R., Teynor, T. M., Doll, J. D., and Durgan, B. R. 1991. Mustard.

Pages 1­- ­6 in: Alternative Field Crops Manual. University of Wisconsin Cooperative Extension, Madison, WI. Peirce, L. C. 1987. Vegetables: Characteristics, Production, and Marketing. John Wiley and Sons, New York.

Pouzet, A. 1995. Agronomy. Pages 65­-­92 in: Brassica Oilseeds­— ­Production and Utilization. D. Kimber and D. I. McGregor, eds.

CAB International, Wallingford, Oxon, U.K. Rubatzky, V. E., and Yamaguchi, M. 1997. World Vegetables­— ­Principals, Production, and Nutritive Values, 2nd ed. Chapman & Hall, New York.

Ryder, E. J. 1979. Leafy Salad Vegetables. AVI Publishing, Westport, CT. Wittwer, S. 1987. Chinese cabbage­—­Year­-­round. Pages 271­-­277 in: Feeding a Billion: Frontiers of Chinese Agriculture. S. Wittwer, Y. Yu, H. Sun, and L. Wan, eds. Michigan State University Press, East Lansing, MI.

(Prepared by S. B. Sterrett, G. E. Welbaum, and A. Bratsch)