Compendium of Turfgrass Diseases, Third Edition

Compendium of Turfgrass Diseases THIRD EDITION

Richard W. Smiley Oregon State University Columbia Basin Agricultural Research Center Pendleton

Peter H. Dernoeden University of Maryland College Park

Bruce B. Clarke Rutgers University New Brunswick, New Jersey

The American Phytopathological Society

Front cover photograph by Tony Hellman, Digital Resolutions Golf Photography, Minneapolis, Minnesota, used by permission Back cover photographs by Patti Ek

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: 2005928471 International Standard Book Number: 0-89054-330-5  1983, 1992, 2005 by The American Phytopathological Society First edition published 1983. Second edition 1992. Third edition 2005 Second printing, 2007 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.

Printed in the United States of America on acid-free paper The American Phytopathological Society 3340 Pilot Knob Road St. Paul, Minnesota 55121, U.S.A.

Preface Since the publication of the first edition in 1983, nearly 40,000 copies of the Compendium of Turfgrass Diseases have been distributed for use in numerous countries on all continents except Antarctica. The second edition has been translated into Chinese, Japanese, and Spanish. In this third edition, the authors have incorporated recent advances in turfgrass pathology to present a new authoritative treatment of turfgrass diseases.

The compendium is intended to serve as a general and practical reference for all those involved in the culture of fine turf. It is designed primarily for those with little training in plant pathology and for pathologists who are inexperienced in diagnosing turfgrass diseases. It will continue to be a valuable resource for professional turfgrass managers; turfgrass management and disease consultants; agribusiness research and sales representatives; officers in departments of agriculture and regulatory agencies; garden center personnel; county extension agents and advisory officers; technicians in plant disease diagnostic clinics; and instructors and students of turfgrass management, landscape architecture, plant pathology, vocational agriculture, and related fields.

This book blends descriptive terminology with the technical language of plant pathology to better serve the diverse levels of knowledge and skills of the intended audience. The text is complemented by numerous illustrations of turfgrass diseases, photomicrographs of many pathogens, keys for identification of diseases and pathogens, diagrams of disease cycles, a glossary of terms, and an index. The book includes sections that briefly describe the characteristics of turfgrasses, the damage caused by noninfectious agents in the turfgrass ecosystem, the ecology and taxonomy of fungi pathogenic to turfgrasses, and elementary diagnostic procedures.

As in previous editions, the emphasis is on diseases occurring in North America, the United Kingdom, and Europe, but not to the exclusion of diseases affecting turfgrasses elsewhere.

Turfgrass diseases are described as they occur in the most common grasses maintained as lawns, sports fields, golf courses, bowling greens, sod production fields, and utility areas along roadways and airport runways. Diseases of inflorescences and culms have been excluded. The control measures described are presented as principles or strategies, because specific measures vary greatly with individual circumstances. Detailed information on currently recommended control methods should be obtained from university turfgrass disease specialists, advisory officers, or crop protection specialists. It is not possible in a book of this type to catalog all appropriate references, but the selected references cited serve as points of access to more comprehensive literature.

About half of the text has been revised since the second edition. The third edition contains 35% more illustrations than the second edition. This revision has benefited from the reviews of many scientists. Each section was reviewed by the three authors, and approximately half the sections were sent to specialists who reviewed them and in some cases provided complete

revisions. Over 40 scientists contributed directly to the information contained in this book.

In her capacity as a senior editor for APS Press, Ann Gould (Rutgers University) was the liaison between the publisher and the authors. Dr. Gould reviewed the manuscript and coordinated the peer review. The editors are deeply grateful for the thorough reviews performed by Richard Latin (Purdue University) and Leon Burpee (University of Georgia). Comments regarding errors or omissions always are welcome. With your suggestions and assistance, future editions can become even more valuable.

Oregon State University, the University of Maryland, and Rutgers University generously contributed the authors’ time and materials for this revision. We especially wish to recognize the following contributors, for their thorough reviews, editing, and, where noted in parentheses, principal authorship of new and revised sections of this book: C. A. Bigelow, Purdue University, Lafayette, Indiana S. A. Bonos, Rutgers University, New Brunswick, New Jersey L. L. Burpee III, University of Georgia, Experiment B. S. Corwin, University of Missouri, Columbia M. L. Elliott, University of Florida, Fort Lauderdale B. I. Hillman, Rutgers University, New Brunswick, New Jersey (Viral Diseases) T. Hsiang, University of Guelph, Guelph, Ontario G. Jung, University of Wisconsin, Madison J. E. Kaminski, University of Connecticut, Storrs R. T. Kane, Chicago District Golf Association, Chicago P. J. Landschoot, Pennsylvania State University, State College I.-M. Lee, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, Maryland S. B. Martin, Clemson University, Florence, South Carolina (Rapid Blight) W. A. Meyer, Rutgers University, New Brunswick, New Jersey N. A. Mitkowski, University of Rhode Island, Kingston J. A. Murphy, Rutgers University, New Brunswick, New Jersey N. A. Tisserat, Colorado State University, Fort Collins L. P. Tredway, North Carolina State University, Raleigh (Pink Patch; Red Thread; Take-All Patch) W. Uddin, Pennsylvania State University, University Park (Gray Leaf Spot) P. Vincelli, University of Kentucky, Lexington T. J. Volk, University of Wisconsin, La Crosse J. F. White, Jr., Rutgers University, New Brunswick, New Jersey H. T. Wilkinson, University of Illinois, Urbana S. J. Zontek, United States Golf Association, Green Section, West Chester, Pennsylvania

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Contents Introduction 2 4 6 6

53 54 56 56 59 62 63 68 70 71 78 85 86 88 89 91 94 97 99 102

Grasses Managed as Turfs Turfgrass Genera Diseases and Their Causal Agents Infectious Agents

Part I Noninfectious Diseases 8 8 8 9 9 11 11 13 15

Biotic Agents of Noninfectious Diseases Algae Black-Layer Moss Insect Pests Abiotic Agents of Noninfectious Diseases Chemical Agents Physical Agents Mechanical Agents

Part II Infectious Diseases Caused by Fungi 17 17 19 19 19 20 21 22 24 28 28 31 33 33 34 34 35 37 37 38 40 44 46 47 47 49 50 53

Tar Spot Yellow Tuft (Downy Mildew) Fungal Diseases of Foliage and /or Roots Anthracnose Bipolaris and Exserohilum Diseases Curvularia Diseases Drechslera and Marielliottia Diseases Fusarium Diseases Nigrospora Blight Pythium Diseases Rhizoctonia Diseases Seedling Diseases Southern Blight Fungal Diseases of Roots Dead Spot Necrotic Ring Spot Root Decline of Warm-Season Turfgrasses Spring Dead Spot Summer Patch Take-All Patch

Part III Diseases and Disorders Caused by Other Pathogens and Biotic Agents

Fungal Diseases of Foliage Ascochyta Leaf Blight Brown Stripe Cephalosporium Stripe Cercospora Leaf Spot Cladosporium Eyespot Copper Spot (Zonate Leaf Spot) Dollar Spot Gray Leaf Spot Leaf Blotch (Scald) Leaf Smuts Leptosphaerulina Leaf Blight Mastigosporium Leaf Spot Phyllosticta Leaf Blight Physoderma Leaf Spot and Leaf Streak Pink Patch and Cream Leaf Blight Powdery Mildew Pseudoseptoria Leaf Spot Ramularia Leaf Spot Red Thread Rusts Septoria Leaf Spot and Stagonospora Leaf Spot Snow Molds Coprinus Snow Mold Microdochium Patch Snow Scald Typhula Blight Spermospora Leaf Spot

106 109 110 114 114 115 116 117 122 123 125 126

Bacterial Diseases Endophytic Fungi Fairy Ring Mollicute Diseases Superficial Fairy Ring Yellow Ring White Blight Diseases Caused by Plant-Parasitic Nematodes Primitive Root-Infecting Fungus-Like Organisms Rapid Blight Slime Molds Viral Diseases

Part IV Ecology and Taxonomy of Pathogenic Fungi 130 Ecological Groupings of Pathogens and Parasites 133 Pathogens and Their Environment 133 Pathogen Successions and Disease Complexes 138 Taxonomy of Pathogens and Other Agents Causing Diseases of Turfgrasses v

Part V Disease Control Strategy

Part VI Disease Diagnosis

139 139 140 142 145 146

148 Diagnostic Procedures 150 Guide to Turfgrass Diseases and Disease Groups

Seedbed and Seed, Sprig, or Sod Sanitation Disease-Resistant Species and Cultivars Management of Turfgrass Climate and Culture Chemical Control Nontarget Effects of Pesticides Biological Control

155 Glossary 163 Index

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Compendium of Turfgrass Diseases THIRD EDITION

Introduction Turfgrasses have been recognized for their importance to the quality of life for 2,000 years. Historical records establish that turfgrasses grew in the vast palace gardens of the emperor of the Han dynasty in China (100 b.c.); in the Persian garden carpets in the kingdom of Assyria (a.d. 500); on the sports fields of Akbar the Great, emperor of Hindustan (a.d. 1600); and in the lawn gardens of Britain in the thirteenth century.

Today, turfgrasses are cultured in nearly all inhabited regions of the world. Turf species and cultivars of the family Poaceae (Gramineae) are remarkably adaptable, some of them having adapted to subarctic regions and others to equatorial regions.

Turfgrasses serve us in many important ways. As ornamental plants, they add beauty to the environment and improve the aesthetic value of our lives. By serving as playing fields for many sports, they provide for recreational needs and help to limit injuries common to vigorous sports. Our leisure time is greatly enhanced by well-­maintained turf.

Perhaps the least apparent way in which turf fills a need in our lives is in its functional role. Turfgrass is widely used to control water, sediment, and wind erosion. It provides a utilitarian cover around houses and public and commercial buildings and in parks, cemeteries, and other facilities. Turfgrass adds value to property and reduces air and noise pollution, heat buildup, visual pollution, and glare. Turf is maintained on and around airfields to reduce dust and thereby extend airplane engine life and increase safety. Similar benefits are obtained from turf in safety zones along highways.

The size and value of the turfgrass industry are difficult to determine, since turf is a perennial crop, and once established at a site can remain there for generations. The acreage of turf is generally directly related to population density and to the wealth and leisure time of the population. Over a billion dollars is spent annually in most urban states on salaries, equipment, and supplies for the maintenance of turfgrasses. A survey of the turfgrass industry in New York State (NASS, 2004) revealed that more than five billion dollars was spent in 2003 to maintain 1.6 million hectares (3.4 million acres) of turfgrass in that state. The survey showed that turfgrass covered 10% of the land area of New York and was maintained mostly around private residences (82% of the total area planted with turfgrass), with golf courses (3%), parks (2%), and other amenity uses accounting for smaller portions of the total. Hence, the turfgrass industry is one of the largest segments of agriculture and horticulture in the United States.

Turfgrass plants are affected by numerous pathogenic agents, insects, and physiological disorders. The costs of diseases and of efforts to control them are difficult to evaluate. Pathologists and other turfgrass scientists have often focused on disease control as a primary research objective. Pathologists serve as key members in research institutes dedicated to improving turfgrass culture in many countries. Pathology regularly takes its rightful place in meetings of the International Turfgrass Society and other national and regional societies.

The first monograph on turf diseases and their control was by

Monteith and Dahl in 1932. The science of turfgrass pathology gained more formal recognition with the publication of several reference works, including those by Howard et al. (1951), Smith (1959), Couch (1962), Beard (1973), Vargas (1981), and Smiley (1983). Most of these texts have been revised, and numerous otherbooksandpublicationsonturfgrasspathologyanddisease management are now available. Schumann and MacDonald (1997) produced a CD-­ROM adapted from the Compendium of Turfgrass Diseases as a useful tool for diagnosticians and professional turfgrass managers. The most definitive texts on turfgrass pathology as of this writing are those of Smith et al.

(1989) and Couch (1995). These excellent resources will serve as standard technical references on turfgrass pathology for many years.

The third edition of the Compendium of Turfgrass Diseases is a further extension of knowledge of turfgrass diseases and the taxonomy of fungal pathogens, according to authorities cited by Farr et al. (1989).

Selected References Beard, J. B. 1973. Turfgrass: Science and Culture. Prentice-­Hall, Englewood Cliffs, N.J. Beard, J. B. 2002. Turf Management for Golf Courses. Ann Arbor Press, Chelsea, Mich.

Couch, H. B. 1962. Diseases of Turfgrasses. Reinhold, New York.

Couch, H. B. 1995. Diseases of Turfgrasses. 3rd ed. Krieger Publishing, Malabar, Fla. Farr, D. F., Bills, G. F., Chamuris, G. P., and Rossman, A. Y. 1989.

Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, Minn.

Howard, F. L., Rowell, J. B., and Keil, H. L. 1951. Fungus diseases of turfgrasses. Univ. R.I. Agric. Exp. Stn. Bull. 308.

Larsen, P. O., and Joyner, B. G., eds. 1980. Advances in Turfgrass Pathology. Harcourt Brace Jovanovich, Duluth, MN. Monteith, J., Jr., and Dahl, A. S. 1932. Turf diseases and their control.

Bull. U.S. Golf Assoc. Green Sect. 12(4):85–188.

NASS (National Agricultural Statistics Service). 2004. New York turfgrass survey. New York Agricultural Statistics Service, Albany. (http://www.nass.usda.gov/ny/Turfgrass2003/Turfbook04.pdf) Roberts, E. C., and Roberts, B. C. 1989. Lawn and Sports Turf Benefits.

Lawn Institute, Pleasant Hill, Tenn.

Schumann, G. L., and MacDonald, J. D. 1997. Turfgrass Diseases: Diagnosis and Management. CD-­ROM. American Phytopathological Society, St. Paul, Minn.

Smiley, R. W. 1983. Compendium of Turfgrass Diseases. American Phytopathological Society, St. Paul, Minn.

Smith, J. D. 1959. Fungal Diseases of Turfgrasses. Sports Turf Research Institute, Bingley, U.K. Smith, J. D., Jackson, N., and Woolhouse, A. R. 1989. Fungal Diseases of Amenity Turf Grasses. E. & F. N. Spon, London.

Tani, T., and Beard, J. B. 1997. Color Atlas of Turfgrass Diseases. Ann Arbor Press, Chelsea, Mich.

Vargas, J. M., Jr. 1981. Management of Turfgrass Diseases. Burgess Publishing, Minneapolis, Minn.

Vargas, J. M., Jr. 2005. Management of Turfgrass Diseases. 3rd ed.

John Wiley & Sons, Hoboken, N.J.

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Grasses Managed as Turfs The family Poaceae (formerly the Gramineae) is composed of approximately 7,500 species in 600 genera. About 40 species in 19 genera, belonging to the subfamilies Festucoideae, Panicoideae, and Eragrostoideae, are commonly managed as turfgrasses (Table 1). Each subfamily is further subdivided into tribes, seven of which include turfgrasses. Commercial cultivars are derived by natural selection, traditional breeding of parental plants of the same species, and molecular techniques.

As a group, turfgrasses have diverse characteristics (Fig. 1). They are grown around the world, often with widely diver-

gent levels of management. Many important diseases occur in grasses managed as turfs. Diagnosis of turfgrass diseases is expedited by identification of the host species. The following descriptions of turfgrass gener­a and the accompanying key are provided as aids to identification of turfgrasses for the purpose of diagnosis.

Turfgrasses in the subfamily Festucoideae are often referred to as cool-­season grasses, because they are adapted for optimal growth in temperate and subarctic climates. Optimal growth occurs at 60–75°F (15–25°C). Their geographic range is restricted

Fig. 1. Parts of a grass plant. (Reprinted, by permission, from Scotts Information Manual for Lawns, 1979, O. M. Scott & Sons, Marysville, Ohio)

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by their lack of adaptation to prolonged or severe high temperature or drought. These grasses are classified as C3 grasses, because carbon is fixed primarily via the Calvin (C3 ) cycle during photosynthesis. Most turfgrasses in cool, humid climates belong to the genera Agrostis, Festuca, Lolium, and Poa. Limited culture of grasses in the genera Agropyron, Ammophila, Bromus, Cynosurus, Dactylis, Deschampsia, Koeleria, Phleum, and Puccinellia also occurs in these regions.

TurfgrassesinthesubfamiliesEragrostoideaeandPanicoideae are commonly called warm-­season grasses, because they are best adapted for growth in tropical and subtropical climates.

Eragrostoid grasses are also extensively grown in warm temperate climates. Optimal growth of warm-­season grasses occurs at 75–95°F (25–35°C). These grasses are classified as C4 grasses, because carbon fixation during photosynthesis occurs mainly via the dicarboxylic acid (C4 ) pathway. The geographic range 3

of C4 grasses is restricted primarily by their lack of tolerance for prolonged or severe cold. Most turfgrasses in warm climates belong to the genera Bouteloua, Buchloë, Cynodon, and Zoysia (in the subfamily Eragrostoideae) and the genera Axonopus, Eremochloa, Paspalum, Pennisetum, and Stenotaphrum (in the subfamily Panicoideae).

Decumbent broadleaf plants are also used as well-­managed amenityturfsinsomeregions.Thiscompendiumdoesnotinclude diseases of Cotula in New Zealand, Dichondra and Trifolium in North America, and other turfs outside the Poaceae.

Turfgrass Genera Agropyron

Wheatgrasses (Agropyron spp.) are coarse-­textured grasses with high tolerance for cold and drought. Several species are used for low-­maintenance lawns, fairways, and roadside turfs in semiarid temperate regions.

Agrostis

Bentgrasses (Agrostis spp.) are generally low-­growing, finet­ extured grasses. They can make high-­quality greens for golf and bowling. They are very susceptible to diseases, especially whenmanagedwithlowcuttingheights.Bentgrassesareadapted to moderately fertile, acidic soils that drain well. Four species (colonial, creeping, velvet, and dryland bentgrasses) are cultivated as turfgrasses. A fifth species (redtop) is of limited use.

Axonopus

Carpetgrasses (Axonopus spp.) are coarse-­textured, light green, low-­growing grasses, adapted to soils of low fertility in subtropical regions. They are intolerant of low temperatures and drought. Two species of Axonopus are cultivated for use in lawns, along roads, and in other utility areas.

Bouteloua

The genus Bouteloua contains about 50 species, two of which are used as turfgrasses: B. gracilis (blue grama) and B. curtipendula (side-­oats grama).

Blue grama is a fine-­textured grass adapted to alkaline soils of low fertility in semiarid transitional climates. It has good heat and drought tolerance. It is used on roadsides and at other low-­maintenance sites and is sometimes used in secondary roughs on golf courses. Its range is similar to that of buffalograss (Buchloë dactyloides).

Side-­oats grama is occasionally planted as a turfgrass in semiarid regions. It is less tolerant of drought than blue grama.

Unlike blue grama, it has a rolled vernation.

Bromus

Bromegrasses (Bromus spp.) are coarse-­textured grasses planted for soil stabilization on low-­use sites, such as roadsides, in the central and western United States. They have good tolerance to drought, heat, and cold but are intolerant of close mowing or frequent traffic. Two species of Bromus are cultivated as turfgrasses.

Buchloë

Buffalograss (Buchloë dactyloides) is a fine-­textured and strong sod-­forming grass native to North America. It is tolerant of alkaline and droughty soils and extreme cold and heat.

Buffalograss is used as a low-­maintenance turf in semiarid temperate regions. Improved cultivars have been developed for lawns and golf course fairways and roughs.

Cynodon

Bermudagrasses, or couchgrasses (Cynodon spp.), are the most common warm-­season turfgrasses, grown throughout the 4

warm, humid, and dry regions of the world. Their range extends from tropical regions to cool regions. Several species and hybrids are cultured for diverse uses, including golf greens and fairways, lawns, and athletic fields. Bermudagrasses are adapted to a wide range of soil conditions, but they are intolerant of dense shade, extremely cold climates, and very wet soils.

Deschampsia

Grasses in the genus Deschampsia vary from coarse-­ to fine-­textured. Tufted hairgrass (D. cespitosa) performs well as a turf under shade and low-­maintenance conditions. The primary disadvantage of Deschampsia spp. is their susceptibility to damage by insects, especially billbugs (Sphenophorus spp.).

Information on disease susceptibility and stress tolerance in Deschampsia is limited, but rust caused by Puccinia spp. has been reported to be severe in the northeastern United States.

heat, drought, and wear tolerance. Kikuyugrass is found in moist tropical regions of Mexico, Australia, and Africa. It is also found in coastal regions in southern California, where it is often considered a weed but can be managed as turf.

Phleum

Timothy grasses (Phleum spp.) are coarse-­textured and are added to seed mixtures for athletic fields in northern Europe and Asia and for roadsides in temperate regions of the United States. Two species of Phleum are used as turfgrasses.

Poa

Fescues (Festuca spp.) vary from fine-­leaf to coarse-­textured bunch types and sod-­forming species. Fine-­leaf fescues are grown in monoculture or planted in seed mixtures with Agrostis, Poa, Lolium, and F. arundinacea (tall fescue). Fine­leaf fescues grow well at droughty or shaded sites, and on less fertile and more acidic soils, but they do not tolerate low mowing or traffic in hot, dry weather. Tall fescue is a medium-­ to coarse-­textured species that tolerates a wide range of soil conditions and moderate shade and has good wear tolerance. Six species of Festuca are used as turfgrasses.

Bluegrasses (Poa spp.) are the most common turfgrasses in humid temperate regions. Four species are used as turfgrasses.

Kentucky bluegrass, or smooth meadowgrass (P. pratensis), produces a dense, attractive turf that grows poorly on infertile and acid soils and readily becomes dormant during droughty periods. There is great variation in disease susceptibility among ­cultivars.

Annual bluegrass, or annual meadowgrass (P. annua), is widely distributed and often becomes dominant in moist and fertile soils. Annual bluegrass is generally not planted but is managed as a turf in many areas.

Roughstalk bluegrass (P. trivialis) grows well in wet and shadedsites.Itisoverseededinwinter-­dormantbermudagrasson golf courses. In cooler regions, it is generally considered a weed, especially when it is a contaminant in seed lots of other ­grasses.

Texas bluegrass (P. arachnifera) is native to parts of Texas and Oklahoma and is more tolerant of heat and drought stress than Kentucky bluegrass. This species is being bred with Kentucky bluegrass to produce hybrids incorporating the desirable characteristics of both species. When planted as turfgrass, these hybrids may expand the range of Kentucky bluegrass to the southern United States, where better heat tolerance and drought tolerance are needed.

Koeleria

Puccinellia

Eremochloa

Centipedegrass, or Chinese lawngrass (Eremochloa ophiuroides), is widely used as a lawn and utility turf in the sub­ tropics and tropics. It grows well in moist, acid, and infertile soils. It requires little maintenance, but it is intolerant of intensive wear.

Festuca

Junegrass (Koeleria macrantha) is an important native prairie grass in parts of North America. As a turfgrass, it forms a dense, medium green turf. Junegrass performs well under low­maintenance conditions and prefers sandy soils.

Lolium

Perennial ryegrass (Lolium perenne) is extensively grown in monoculture or planted in seed mixtures with Festuca and Poa. It may be overseeded into Cynodon turfs to provide a green cover in winter. Perennial ryegrass tolerates a wide range of soil conditions but requires good soil fertility if it is to produce a high-­quality turf. It may exhibit poor quality during seed head production and in periods of heat and drought stress, and it is intolerant of ice cover. Many diverse cultivars of perennial ryegrass have been developed, and they are susceptible to many diseases.

Italian ryegrass, or annual ryegrass (L. multiflorum), is used mostly in inexpensive seed mixtures to provide a temporary ground cover.

Paspalum

Bahiagrass (Paspalum notatum) is a coarse-­textured grass that grows well on sandy, infertile soils in subtropical regions. It is intolerant of low temperatures and extensive wear. This grass is used mostly as a turf for utility areas, such as ­roadsides.

Seashore paspalum (P. vaginatum) is a dense-­growing, dark green, fine-­textured grass adapted to tropical and subtropical climates. It tolerates higher salt levels than many other turfgrass species.Improved,vegetativelypropagatedcultivarsareusedon golf greens, tees, and fairways where there are salinity problems.

Pennisetum

Kikuyugrass (Pennisetum clandestinum) is a medium-­ to coarse-­textured grass adapted to fertile, moist soils. It has good

Alkaligrasses (Puccinellia spp.) are used as turfgrasses in saline and sodic soils of semiarid temperate regions. They are adapted for growth along roadways, especially where salt is spread for ice removal. Three species of Puccinellia are cultivated as turfgrasses.

Stenotaphrum

St. Augustinegrass (Stenotaphrum secundatum) is coarset­ extured grass widely used in lawns in the subtropics and tropics. It is best adapted for growth in moist and fertile soils and shaded sites. Its zone of adaptation is restricted by its intolerance of cold.

Zoysia

Zoysiagrasses (Zoysia spp.) are medium-­textured, heat- and drought-­tolerant grasses that can survive in most climatic regions from the tropics to the subarctic. There is considerable variation in cold tolerance and leaf texture among Zoysia spp. In general, growing regions for zoysiagrasses are limited to warm temperate and warm humid climates. Zoysiagrasses tolerate poor soils but are slow to become established. Three species— ­Z. japonica (Japanese lawngrass), Z. matrella (Manilagrass), and Z. tenuifolia (Mascarenegrass)—­and two hybrids are cultivated as turfgrasses.

Selected References Beard, J. B. 2002. Turf Management for Golf Courses. 2nd ed. Ann Arbor Press, Chelsea, Mich.

Busey, P. 1989. Progress and benefits to humanity from breeding warm-­season grasses for turf. Pages 49–70 in: Contributions from Breeding Forage and Turf Grasses. Crop Science Society of America, Madison, Wisc.

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Christians, N. 1998. Fundamentals of Turfgrass Management. Ann Arbor Press, Chelsea, Mich.

McCarty, L. B. 2004. Best Golf Course Management Practices. 2nd ed. Prentice Hall, Upper Saddle River, N.J. Meyer, W. A., and Funk, C. R. 1989. Progress and benefits to humanity from breeding cool-­season grasses for turf. Pages 31–48 in: Con­

tributions from Breeding Forage and Turf Grasses. Crop Science Society of America, Madison, Wisc.

Turgeon, A. J. 2005. Turfgrass Management. 7th ed. Prentice Hall, Upper Saddle River, N.J. Walker, C., ed. 1971. Turf Culture. New Zealand Institute for Turf Culture, Palmerston North.

Diseases and Their Causal Agents When a pathogenic organism or an environmental factor causes an abnormal alteration in the physiological processes and morphological development of a plant, the plant is considered to have a disease. Because plants are highly responsive to environmental extremes, a healthy plant must be fully adapted to the region where it is grown and to the use for which it is intended.Idealconditions are seldom achieved, however,because many factors may alter the manufacture, translocation, and utilization of plant photosynthates, mineral nutrients, and water.

Agents of disease (or, broadly, any deleterious condition) can be classified as either infectious or noninfectious. Infectious (biotic) agents include plant-­pathogenic viruses, mollicutes, bacteria, fungi, and nematodes (Fig. 2). Noninfectious (abiotic) agents include forces causing mechanical injuries and severe deviations in chemical and physical properties of air (temperature, water, gases, pollutants, light) or soil (temperature, water, gases, pH, compaction, foreign chemicals). These noninfectious agents cause physiological disorders resulting from extremes in the conditions necessary for life.

Noninfectious agents are independently capable of ­causing

disease, but they may also cause plants to become more susceptible to attack by infectious agents. Noninfectious and infectious agents can act in unison to cause complex, poorly defined diseases. In such instances, it may become necessary, if practical, to manage noninfectious agents before or at the same time as managing the infectious agents. Management of noninfectious agents of disease often helps to prevent the occurrence of damaging levels of infectious diseases. For this reason, the control strategies discussed in this compendium emphasize the prevention of environmental and cultural conditions that predispose plants to infection by pathogens.

Infectious Agents Viruses

Viruses are tiny nonliving particles consisting of nucleic acid (RNA or DNA) with a protein coat. They measure only a fraction of one micrometer and can be observed only through an electron microscope. All viruses are parasitic and multiply only in living cells. They cause disease by upsetting the metabolism of plant cells; the metabolic disorder may in turn cause cells to produce abnormal and injurious substances. Viruses are transmitted from plant to plant primarily by the feeding activities of insect or nematode vectors. The severity of virus diseases in grass can be reduced, but not eliminated, by controlling the activities of their vectors.

Viroids

Viroids are infectious molecules consisting only of nucleic acid. They are much smaller than viruses, but they act like viral agents. None have been described as pathogens of grasses.

Bacteria

Bacteria are tiny prokaryotic organisms that contain cytoplasm enclosed by a cell wall but lack organized nuclei. They do not require a living host for replication and growth. Most bacteria are saprophytes, but a few are infectious agents. The pathogenic species are short and rod-­shaped, about 0.5 × 2.0 µm. They have no means of penetration of plants, and thus must enter a host through natural openings or through wounds created by insects, animals, or machines. Infected plants display a wide range of symptoms. Bacterial diseases of grasses are best controlled by planting resistant species or cultivars.

Mollicutes

Fig. 2. Relative sizes of plant pathogens and a host plant cell.

(Reprinted, by permission, from M. V. Wiese, 1977, Compendium of Wheat Diseases, American Phytopathological Society, St. Paul, Minn., as adapted from G. N. Agrios, 1969, Plant Pathology, Academic Press, New York)

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Mollicutes are prokaryotic organisms, including mycoplasmas, phytoplasmas, and spiroplasmas, in which the cytoplasm is bounded by a membrane, without true cell walls. They are members of the Kingdom Bacteria (Monera) and are also known as tenericutes. Mollicutes are found in the phloem of plants. They are transmitted to plants by the feeding activities of leaf­hoppers and certain other insects. Mollicutes are known to occur in grasses, but they are seldom detected as agents of turfgrass diseases. Mollicutes can be controlled with tetracycline anti­biotics or by limiting transmission by insect vectors.

Rickettsias

Rickettsias are prokaryotes that resemble bacteria in most characteristics. These organisms are found in the phloem or xylem of certain plants. They are transmitted by leafhoppers that have fed on infected plants. Rickettsias can attack some grasses, but they are not known to cause diseases of turfgrasses and are not considered further in this book.

Protists

Protists are primitive eukaryotic fungus-­like organisms that do not fit clearly into the fungal, plant, and animal kingdoms.

These organisms are grouped in the Kingdom Protoctista.

They are not members of the Kingdom Fungi, in part because they produce motile zoospores with flagella. Protists are uni- or multicellular and have organelles and a membrane-­bound nucleus. Some protists are important pathogens or parasites of turfgrasses: oomycetes, such as Pythium and Sclerophthora; root-­infecting organisms such as Polymyxa, which are vectors of plant viruses; and the slime molds and a net slime mold.

While protists do not fit the strict taxonomic definition for members of the Kingdom Fungi, they conform to the practical concept of fungi, and they are considered fungi for the purposes of this compendium.

Fungi

Fungi are microscopic organisms that lack chlorophyll and, therefore, sustain life by feeding on living or dead plants or animals. They are multicellular and usually filamentous, and they have well-­developed cell walls, nuclei, and reproductive systems. Some fungi are almost as small as bacteria; others, such as mushrooms, are clearly visible. Fungi can be transported over long distances by insects, animals, and machinery and in

plant debris or soil carried by wind and water. Many can grow over short distances on plants or through soil. Most recognized turfgrass diseases are caused by fungi, and the structure and biology of these organisms are discussed in more detail in the descriptions of diseases that follow. Diseases caused by fungi can be controlled or reduced by planting resistant grass species or cultivars, by proper management practices, and by treatment with fungicides.

Nematodes

Nematodes are tiny but complex animals (roundworms) that live primarily as saprophytes on dead plants or animals. Some parasitic species, however, feed on roots of plants, including turfgrasses, puncturing root cells with their needle-­like mouthparts. Nematodes are abundant and cause various types of root dysfunction. They also cause wounds that enable fungi and other infectious agents to enter plants. These animals are important agents of disease in turfgrasses.

Protozoa

Protozoa are single-­celled, microscopic animals that cause several diseases of plants, but none is known to occur in grasses.

Selected References Agrios, G. N. 2005. Plant Pathology. 5th ed. Academic Press, San Diego, Calif.

Holliday, P. 1989. A Dictionary of Plant Pathology. Cambridge Uni­ versity Press, Cambridge.

Money, N. P. 1998. Why oomycetes have not stopped being fungi.

Mycol. Res. 102:767–768.

Ulloa, M., and Hanlin, R. T. 2000. Illustrated Dictionary of Mycology.

American Phytopathological Society, St. Paul, Minn.

7

Part I. Noninfectious Diseases Biotic Agents of Noninfectious Diseases Turfgrasses are often damaged by the feeding activity of insects and by competition from weeds, moss, and algae. None of these pests is considered to cause true disease, but the disorders they cause may mimic disease. Insects that feed on crowns, roots, or stem bases or extract plant sap from foliage may cause plants to appear diseased. Careful diagnosis is required to assess the true cause of the damage. The task can be difficult if infectious agents become established in wounds created by insects or mechanical damage; in such cases, the primary cause of the disorder may rapidly become masked. Weeds, moss, and algae are usually quick to occupy voids that remain after turf has been thinned by infectious diseases or environmental stresses. Plants that invade weakened turf often mask the initial problem. Pesticides and management practices can control or reduce damage from most noninfectious pests.

algal growth and complement the recovery of grasses when wet conditions also are corrected.

Black-­Layer

Blue-­green algae (cyanobacteria) develop as a black scum on the surface of overly wet soils, particularly on golf and bowling greens (Figs. 3 and 4). The algae may reduce the exchange of gases between air and soil, which may induce chlorosis in plants and thinning of the stand.

Algal scums can be controlled by improving soil and air drainage as well as irrigation procedures; maintaining proper soil pH and nutritional levels; increasing the mowing height; and increasing the penetration of sunlight through shrubs and trees. Some fungicides and other chemicals help to alleviate

Black-­layer is a physical condition of soil, primarily associated with golf greens. Although it occurs in turfs grown on native soils, it is especially noted in turfs grown on substrates with high sand content. The major predisposing factor for black-­layer is anaerobic conditions (lack of oxygen) in the soil.

Algae may play a role in the development of some black layers by plugging pore spaces or sealing the surface.

When soil becomes anaerobic, turf first develops a yellow or bronzed appearance and later thins in irregular patterns, especially in low-­lying or shaded areas where air circulation is poor.

Surface or subsurface black layers (Fig. 5) occur in the sand profile or in the thatch. Subsurface layers may develop 0.5–3.0 in. (1–8 cm) below the surface. Bands of blackened sand are usually 0.25–2.0 in. (0.6–5.0 cm) deep but have been observed as much as 7 in. (18 cm) deep. A foul, sulfurous, rotten egg odor is sometimes associated with affected soils.

Black-­layer is commonly associated with periods of excessive rain or irrigation, especially on golf and bowling greens with poor internal soil drainage or with a high water table.

It often occurs where a sand topdressing is layered over a heavier soil or a thatch layer. Installing unwashed sod can also create soil layers that form a perched water table. Soils of low oxygen content are invariably associated with black-­layer.

Fig. 3. Thin algal scum on a golf green. (Courtesy R. W. Smiley)

Fig. 4. Thick algal crust. (Courtesy M. C. Shurtleff, photo by M. P. Britton)

Algae

8

However, there are no other factors common to all affected turfs.

Under anaerobic conditions, sulfur combined with organic matter is converted by sulfate-­reducing bacteria to hydrogen sulfide gas, which is directly toxic to roots. Blackening results when this gas combines with iron or other divalent cations in soil to form a sulfide precipitate. Blue-­green algae may also contribute to the development of black-­layer. The algae are filamentous and have a mucilage cover, and algal filaments can slowly plug pore spaces in sand. Roots are killed by lack of oxygen, hydrogen sulfide gas, and related stresses of anaerobiosis.

Black-­layer is controlled by eliminating, avoiding, or minimizing perched water tables and other causes of soil waterlogging, such as excessive irrigation, continually irrigating to the same depth, soil compaction, and plugged or poorly designed subsurface drainage systems. Use sand of proper particle sizes to construct greens and in topdressing mixes. Avoid topdressing materials that are not identical to the greens construction mix.

Frequent core or quadratine aerification, spiking, water injection aeration, and fans help to alleviate anaerobic conditions (Fig. 6).

Physical barriers that restrict airflow, such as trees and brush surrounding affected greens, should be pruned or removed.

Avoid application of fertilizers, pesticides, and soil amendments, and avoid water sources that are alkaline (pH 7.0 and above) or contain significant amounts of sulfur, iron, magnesium, manganese, or organic matter (such as pond water or water containing sewage effluent). Nitrate forms of fertilizer may help to alleviate black-­layer, but they can increase the severity of some fungal diseases. Increasing the mowing height reduces the penetration of light and reduces the formation of algal scums.

Fig. 5. Black-layer in a golf green. (Courtesy P. H. Dernoeden)

Fig. 6. Black-layer in a golf green. The breaks in the dark layer are due to improved aeration following mechanical removal of cores.

(Courtesy P. H. Dernoeden)

Moss Mosses are small plants that can out-­compete with turfgrass and overgrow it (Figs. 7 and 8) when wetness, shade, low mowing,andpoornutritionfavorthegrowthofthemossmorethanthe growth of the grass. Turfgrass growth can be enhanced by raking to remove moss, increasing the mowing height, and improving soil drainage, soil fertility, soil pH, irrigation, the penetration of light through shrubs and trees, and the movement of air over the grass. Chemicals that suppress moss growth are available.

Insect Pests Direct damage by insect pests can easily be confused with some diseases of turfgrasses. Accurate diagnoses of insect and disease problems are essential for selecting proper control methods. Insect pests of turfgrasses can be divided into soil­inhabiting types and leaf- and stem-­inhabiting types.

Soil-­Inhabiting Insect Pests

The larvae of soil-­inhabiting insect pests feed on turfgrass roots and stem bases, causing turf to wilt or die in small to large, irregularly shaped patches. Roots and stem bases should therefore be closely observed when plants are examined.

Grubs (Fig. 9), or larvae of beetles (species of Aphodius, Ataenius, Cotinis, Cyclocephala, Exomala, Maladera, Phyl­ lophaga, Popillia, Rhizotrogus, and other genera), feed on roots. Affected turf (Fig. 10) can sometimes be peeled back as if it were newly laid sod. Birds, skunks, and moles can devastate a turf by digging it up to feed on grubs.

Fig. 7. Moss in shaded, nutrient-deficient turf. (Courtesy R. W. Smiley)

Fig. 8. Moss in turfgrass. (Courtesy J. E. Kaminski)

9

Mole crickets (Scapteriscus spp.) and ground pearls (Mar­ garodes spp.) also feed on roots (Fig. 11) and cause severe damage in warm regions (Fig. 12).

Larvae of billbugs (Sphenophorus spp.), Ataenius beetles, and other species also feed on turfgrass stem bases or roots or both. Affected plants appear weak or wilted and may be severed at the soil line.

Larvae of these soil inhabitants may be found among roots.

The presence of a large population of insects generally indicates thattheyaretheprimarycauseofthesymptomsdescribed above.

Facultatively parasitic fungi usually colonize weak, dying, or dead tissues, and their presence complicates the diagnostic ­process.

Adult and larval stages of thatch- and soil-­inhabiting insect pests also damage turf (Figs. 13–15), especially during warm, dry periods in summer. Larvae (Fig. 16) of burrowing sod webworms (species of Acrolaphus, Crambus, and other genera), cutworms (species of Agrotis, Nephelodes, and other genera), and armyworms (species of Pseudaletia and Spodoptera) chew on leaf blades or sever stems at the soil line. Some larvae construct tunnels in thatch and soil, and most leave green or tan pellets of excrement (frass). Flocks of birds may feed actively on larvae and probe holes in the thatch and soil with their beaks.

Larvae of the annual bluegrass weevil (Listronotus sp.) and billbug larvae feed on stems but leave no tunnels.

Fig. 9. Grubs of the European chafer. (Courtesy A. M. Petrovic)

Fig. 12. Damage caused by ground pearls in Eremochloa ophi­ uroides. (Courtesy L. T. Lucas)

Fig. 10. Damage caused by European chafer grubs. (Courtesy R. W. Smiley)

Fig. 13. Damage caused by sod webworms in Poa pratensis. (Cour­ tesy R. W. Smiley)

Fig. 11. Ground pearls feeding on roots of Eremochloa ophiuroides. (Courtesy L. T. Lucas)

Fig. 14. Damage caused by the annual bluegrass weevil on Poa annua. (Courtesy R. W. Smiley)

10

Fig. 15. Damage caused by chinch bugs and Japanese beetle grubs in mature Poa pratensis. (Courtesy R. W. Smiley)

Fig. 16. Silver-striped webworm. (Courtesy H. Tashiro)

Chinch bugs (Blissus spp.) cause grass in large, scattered patches to turn yellow, wilt, and die during droughty periods in summer (Fig. 15). Chinch bugs inject toxic saliva into leaves and stems as they feed, causing turf to appear chlorotic.

Advanced damage appears like drought injury, but the turf does not recover after rain or when watering resumes.

Leaf- and Stem-足Inhabiting Insects

Leaf-足and stem-足inhabiting insects suck fluids from leaf cells.

Feeding by mites causes a silvery or frosted appearance in affected leaves. The damage may lead to plant desiccation and death. Winter grain mites (Penthaleus major) are black and feed during late winter, causing damage identical to winter desiccation. Clover mites (Bryobia praetiosa) are reddish brown and feed during spring and autumn. Bermudagrass and zoy-

Fig. 17. Greenbugs on a leaf blade. (Courtesy D. Potter)

Fig. 18. Greenbug damage in shady areas under trees. (Courtesy D. Potter)

siagrass mites (Eriophyes spp.) feed during summer and cause distorted growth in the form of a tuft.

Greenbugs (Schizaphis graminum) (Fig. 17) feed during spring, summer, and autumn, sucking fluids from leaves. As the greenbug feeds, a toxic saliva enters the leaf, causing chlorosis in an irregularly shaped area around the feeding site. Leaves eventually turn brown and die. The damage initially occurs in shady areas under trees (Fig. 18) and may then spread outward to larger areas. Greenbug damage is easily confused with competition between tree roots and turfgrasses for a limited supply of water during droughts.

The Rhodesgrass mealybug (Antonina graminis) and the bermudagrass scale (Odonaspis ruthae) suck juices from lower stem nodes or the crown, causing weakening, chlorosis, or wilting of foliage.

Abiotic Agents of Noninfectious Diseases Turfgrasses may be harmed directly by various chemical, mechanical, and physical conditions in the plant environment.

Chemical causes of abiotic damage include pesticides, animal urine or salts, saline or sodic soils, excessive or deficient soil fertility, gaseous pollutants, and chemicals released in spills.

Physical agents include extremes of temperature and of availability of water, compaction, thatch, shading, excessively wet soil, and roots ornamentals and trees in competition with the turf. Mechanical agents include mower shredding or scalping of leaves, frost heaving or bruising of frozen plant tissues, and

abrasive wear. These abiotic factors may interact with infectious agents, making diagnosis difficult.

Chemical Agents Pesticides

Plant health aids should always be applied precisely according to the directions on their containers. Some pesticides may 11

havenontargeteffectsontheturfgrassecosystem(seeNontarget Effects of Pesticides, in Part V). Excessive applications, especially of herbicides, may kill turfgrass plants. Several herbicides and fungicides have growth-­regulating properties that may adversely modify grass growth when they are not used in accordance with the label directions. Any pesticide that is intended to be washed off the foliage must be applied so as to achieve maximum efficiency and avoid killing or damaging foliage, especially on warm, dry days.

Symptoms of pesticide damage often appear in the patterns in which a pesticide has been applied, such as broad swaths, narrow streaks, or other regular patterns. Specific symptoms are highly variable, including leaf speckling (Fig. 19), chlorosis, burn, droughty appearance, and death. Symptoms may occur soon after application or may be delayed for several weeks.

Patches caused by salt damage can be confused with several patch-­type diseases caused by infectious agents.

Animal Urine and Salts

Like the chemical excesses discussed above, nutrient deficiencies also cause direct damage to plants. Some soils are deficient in phosphorus and potassium. Micronutrient deficiencies can occur in turf grown in sandy soils or sand mixes used on golf greens or athletic fields. Apply lime, sulfur, or acidifying sources of nitrogen, such as ammonium sulfate, according to soil test recommendations, to maintain pH in the range of 6–7.

Sulfur should be used with caution, as it can be phytotoxic, especially on golf greens. A soil test should always be used for guidance in alleviating nutrient deficiencies and extremes in soil alkalinity or acidity.

Animal urine contains soluble salts, urea, and other compounds. It causes little damage to turf if it is uniformly dispersed and has a low soluble salt content or if the soil is moist and fertile. Local stimulation of leaf growth and dark green coloration may be apparent from nitrogen released from the urea.

However, if the salt content is high and the urine is deposited in one spot, the affected turf may be killed (Fig. 20), especially in dry or infertile soil. A margin of dark green, rapidly growing grass may surround the dead area. Other sources of salts, such as cooling water from homemade ice cream, fertilizer spills, and materials spread on highways and sidewalks to melt snow and ice (Fig. 21), can also kill turf. Only animal urine causes dead patches with marginal areas exhibiting dark green, stimulated growth. Heavy watering helps to disperse salts and lower their concentration.

Fig. 19. Phytotoxic response to nightly frosts in turf treated with the herbicide oxadiazon. (Courtesy R. W. Smiley)

Fig. 20. Damage caused by dog urine. (Courtesy R. W. Smiley)

12

Fertilizers

Excessive use of fertilizer can damage grasses by disrupting nutrient balances in the plants and inducing excessive growth, which may be subject to scalping during mowing. Many types of fertilizers applied to the foliage of wet grass can cause salt burn. Watering turf thoroughly after fertilizer application is recommended, particularly after application of water-­soluble nitrogen fertilizers having a high salt index. Natural and many synthetic organic fertilizers generally have a low burn potential. Fertilizers should be applied uniformly (Fig. 22) and during the proper season for the type of turf being grown.

Nutrient Deficiencies

Air Pollution

Gaseous products of combustion and manufacturing processes undergo photochemical transformations during transport in the atmosphere. These air pollutants may accumulate

Fig. 21. Damage caused by salt applied to a roadway to melt snow and ice. (Courtesy R. W. Smiley)

Fig. 22. Improper application of fertilizer to turfgrass. (Courtesy O. M. Scott & Sons)

in concentrations high enough to cause toxicity in plants, thereby altering plant metabolism. Air pollutants are usually found in highest concentrations in and near metropolitan areas, manufacturing centers, and energy-­generating facilities, such as fossil-­fuel-­burning power plants. Symptoms of air pollution toxicity in turfgrasses include bleaching, chlorosis, necrosis of leaf tips or leaf margins, formation of white or yellow bands across leaf blades, brown stippling of leaves, and glassy brown discoloration of leaves followed by necrosis; some toxicity causes reduced growth with no visible symptoms of damage.

Pollutants include ozone, peroxyacetyl nitrates, sulfur di­oxide, nitrogen dioxide, hydrogen fluoride, chlorine, hydrogen chloride, ethylene, and toxic dusts. Leaks in fuel oil tanks, gas distribution lines, and other sources may also cause localized damage. Ozone (a photochemical oxidant) and other oxides are the most common pollutants in eastern North America, especially from Massachusetts to North Carolina.

Turfgrass species and cultivars vary greatly in symptom expression and growth responses to air pollutants. Some cultivars have been designated as more suitable than others for growth in industrial and urban areas with poor air quality.

Chemical Spills

Spills of chemical fuels or lubricants, pesticides, soaps, or cleaning products cause severe and often long-­lasting damage to grasses. Injury appears as dead patches, for example, where a fuel or sprayer filter or line was changed (Fig. 23), where a fuel tank was filled, or where a household floor cleaner was discarded. Streaks may also occur where hydraulic fluids, oils, or fuels escape from moving equipment. Some contaminants can be difficult to recognize, such as salts in topsoil, persistent herbicides or other chemicals in soils, and pesticides transported by water to low-­lying areas. Damage can often be minimized when spills are detected quickly and the area is thoroughly washedwithdetergentsandthentreatedwithanabsorbentsubstance, such as activated charcoal.

and death can occur if ice crystals form and disrupt cell walls and membranes. Ice encasement can be especially damaging.

The extent of the damage depends on the rate and frequency of freezing and thawing, the hydration status of the plant tissues, and the soil type.

High-­temperature damage to grasses may result from direct solar radiation (Fig. 25); engine exhaust (a combination of heat and gaseous pollutants); contact with objects such as containers of hot liquids, motors, automotive floor mats, glass, or metal pails in full sunlight; and lightning strikes. Heat is most detrimental to grass when the plants are unable to cool themselves by transpiration. Transpirational cooling is reduced under very high atmospheric humidity, and under dry soil conditions (Fig. 26), or when grass blades are completely covered by a hot object. Direct solar radiation also increases respiratory activities of plant roots and soil microorganisms, which may lead to an accumulation of toxic gases or lethal temperatures in the root zone, especially in wet and compacted soils. Turfgrass root growth is slowed or stopped when soil temperatures are too high for a particular species. Damage due to high air or soil temperatures is more likely to occur in cool-­season grasses than in warm-­season grasses.

Water and Ice

Extremes in the availability of water probably cause more problems than any other single abiotic agent, but the effects of water and temperature are difficult to separate. The importance of transpirational cooling and other interactions were mentioned above. Lack of water restricts plant growth and makes plants more susceptible to mechanical injury and heat damage.

Physical Agents Temperature

Extremely high or low temperatures can directly damage grasses. The overall effect of extremes of temperature depends on the physiological status of the plants at the time of exposure.

Winterkill may be indirect (desiccation) when unfrozen plants are in frozen soil without a protective cover of snow or mulch (Fig. 24). The degree of exposure of a turf area to winter winds and sun can affect desiccation. Direct tissue damage

Fig. 23. Phytotoxicity from an ethazole fungicide spilled around a catch bucket when a sprayer screen was being cleaned. (Courtesy R. W. Smiley)

Fig. 24. Desiccation caused by winds blowing over a frozen golf green. (Courtesy R. W. Smiley)

Fig. 25. Diurnal pattern of bands due to heat stress, on leaves exposed to intense solar radiation and high humidity. (Courtesy R. W. Smiley)

13

Drought stress is often more pronounced next to sidewalks, driveways, and buildings; at the tops of ridges and slopes; and in areas where objects such as large rocks, concrete and other construction debris, or septic tanks are buried. In some turfs, hydrophobic (water-­repellent) soil conditions develop as a result of microbial activity. Areas in which such conditions develop are called localized dry spots (Fig. 27).

Excessive water, combined with poor drainage, creates an unfavorable soil environment for root growth. The lack of oxygen, the accumulation of toxic gases, and supraoptimal tem-

peratures in soil may weaken or kill plants directly. Excessive humidity reduces transpiration and can lead to increases in leaf temperatures, which may be harmful to enzymatic reactions necessary for growth. Turfgrasses inundated by water on hot, sunny days are subjected to extreme heat; the resulting injury is called scald (Figs. 28 and 29).

Prolonged ice cover can cause suffocation (anoxia) of turfgrass (Fig. 30), although respiratory processes are greatly reduced during very cold weather. Under the ice, oxygen is depleted by microbes and plants, and an anaerobic condition develops as a result. Anoxia kills plants directly or predisposes them to freeze injury. Permeable covers are used on golf greens in cold regions to protect the turf from desiccation and freeze injury. Excessive soil wetness can be controlled by proper surface drainage, installation of subsurface drainage systems, and proper irrigation.

Shallow Soil

Fig. 26. Dormant unirrigated turf during a drought. (Courtesy R. W. Smiley)

Soil overlying shallow rock outcroppings or buried objects dries rapidly, and plants growing in it may show drought stress much sooner than those in nearby, deeper soils (Fig. 31). Patches or streaks of wilted turf may appear on gravel or rocky soils. Turf on one residential lot may become dry much sooner than that on neighboring properties if the depths of excavation and topsoil refill differ significantly. Causes of such patches or patterns can be determined by probing the soil profile. Concrete, bricks, lumber, or large rocks buried during construction projects should be removed to alleviate persistent ­problems.

Fig. 27. Localized dry spots on a golf green. (Courtesy P. H. Dernoeden)

Fig. 29. Putting green turf killed by scald following a heavy rain during summer. (Courtesy P. H. Dernoeden)

Fig. 28. Selected grass clones killed by flood stress when waterlogged soil was warmed by an early spring heat wave. (Courtesy R. W. Smiley)

Fig. 30. Grass that suffocated under a sheet of ice. Note the cracking pattern. (Courtesy R. W. Smiley)

14

Soil Compaction

Heavily trafficked turfgrass areas, especially those on overly wet or fine-­textured soils, can become so compacted that normal gas exchange and root growth are restricted (Fig. 32). Such areas include footpaths, animal paths, and places where heavy mowers or vehicles always follow the same routes.

Compaction is difficult to correct without major renovation.

The goals of corrective measures are to alleviate causal factors, to improve gas exchange, and to improve penetration of the soil by water and roots. Traffic over affected areas should be re­directed or restricted. The soil can be modified to include a higher proportion of sand, as specified by a soil particle analysis test. Effective soil modification is difficult to achieve and generally is expensive. Aerification machines that remove many soil cores to a depth of 2 to 4 in. (5–10 cm) or more are particularly useful in alleviating compaction.

Thatch

Thatch is a layer of plant litter that develops above the soil surface, which is the result of an accumulation of dead plant roots, crowns, rhizomes, and stolons. Thatch is not markedly influenced by the removal or return of leaf clippings to turf during mowing. Clippings decompose rapidly and recycle nutrients back to living plants and to microorganisms that decompose plant litter. Thick layers of thatch (Fig. 33) reduce the movement of water, air, fertilizer, and pesticides into the soil and the movement of carbon dioxide and other gases out of the soil. Stems and roots tend to grow in thatch rather than down into the underlying soil, and this increases the risk of damage to plant tissues due to drought and high temperature.

Thatch depths may be reduced by using vertical cutting (dethatching)equipmentandcoreaerificationequipmentwhenturf is actively growing and by applying recommended topdressing sands or soils. A long-­term program of managed microbial decomposition, including precise control of water, pesticides, soil pH, and fertilizers, will help to reduce the rate of thatch development.

Trees and Shrubs

Ornamental plants and trees with shallow roots or dense cano­ pies compete with turfgrasses for water, nutrients, and light.

Damage can be reduced by rearranging species of woody ornamentals, turf, or both. Water competition can be reduced by irrigating the turf more deeply and frequently and by pruning tree roots. In general, turf growing in shaded sites should be mowed higher and requires less nitrogen fertilizer than turf grown in the full sun. Trees should be fertilized with special root-­feeding equipment and products. Problems due to competition for sunlight can be reduced by planting shade-­tolerant grass species or cultivars, increasing the mowing height in shaded areas, selective pruning or removal of trees and shrubs, or exchanging turfgrass for more shade-­tolerant ground covers.

Mechanical Agents Turfgrasses are injured when mowing equipment is dull or poorly adjusted; when the mowing height is too low or mowing is too infrequent; when grass on mounds or on uneven areas is cut too low; when people, animals, or equipment move across drought-­stressed, excessively wet, or frozen turf; and when turf is subjected to excessive use.

Mower Injury

Dull mower blades tear or shred grass leaves (Fig. 34) rather than cut them. When a rotary mower with dull blades is used, vascular bundles are left protruding from grass leaves, giving the turf a grayish cast. This shredding causes the tips of injured leaves to turn straw brown and die within a few days. Such injuries also allow infectious agents to penetrate plants to cause disease. Dull blades or poorly adjusted mowers cause abraded bandsacrossleafblades.Mowerbladesshouldbesharpenedand adjusted frequently to prevent excessive injury to grass leaves.

Scalping Fig. 31. Turf killed by drought above stone blocks buried during construction of a golf fairway. (Courtesy P. H. Dernoeden)

Fig. 32. Turf damaged by compaction of soil in a walkway. (Cour­ tesy R. W. Smiley)

Scalping occurs where grass is mowed so short that yellow or brown stem tissue is exposed. It may occur where a mower wheel drops into a depression, on embankments or other­uneven areas, or in grass that is not mowed frequently enough. Such damage also occurs when the mowing height is lowered too rapidly.

Fig. 33. Excessively thick layer of thatch. (Courtesy R. W. Smiley)

15

Scalping can occur when weedy patches of stoloniferous grasses become established in turf cut at a normal residential lawn height. A patch of weedy grass becomes stringy and fluffy and periodically is lifted and severed by the mower. This leaves a brown area in an otherwise healthy turf.

Scalping injuries to rhizomatous or stoloniferous grasses are often self-­healing, but it may be necessary to reseed damaged areas in bunchgrasses such as Festuca (especially fine-­leaf fescues) and Lolium spp. Patches of undesirable grasses in lawns should be controlled and reseeded or sodded with desirable grasses.

sumes, the damaged leaves may turn brown and fail to grow (Fig. 35). The damage often affects only exposed leaves, but crown damage and plant death can also occur. These injuries occur in recognizable patterns, such as brown, footprint-­shaped marks that develop after walking across turf with heavy frost or drought-­stressed turf. Brown wheel tracks, snowmobile or ski tracks, and the dog’s or the cat’s favorite path around the yard will also appear in frozen, drought-­stressed, or dormant turf.

To remedy this problem, avoid traversing dry turf or frosted turf without snow cover.

Leaf and Crown Bruises

Green grass blades may be injured by excessive use. Such injuries can occur around the bases in backyard ball fields, in the goal areas of soccer and lacrosse fields, in the middle of football fields, at the front lines of a volleyball court, and in the push-­off areas under swings. Mowing the peripheral areas of golf greens or collars during the summer, particularly during wet periods, causes thinning and sometimes death of the turf.

Grass blades in high-­use areas are subject to abrasion, which damages the leaf epidermis. The internal tissues dry quickly, giving the foliage with a dull, gray green appearance that resembles drought stress. The grass fades to a brown or bleached color within several days. The damage may be permanent if the crown is killed or if the turf is completely worn away. Rotating activities to different areas or mowing the peripheral areas of golf greens less frequently promotes regrowth and prevents permanent damage.

When turfgrass plants are frozen or dormant during dry weather, the leaf and crown tissues become brittle and can be damaged by pressure from the wheels of mowers or vehicles and from the feet of people or animals. When plant growth re-

Abrasive Injury

Selected References

Fig. 34. Shredding of leaf blades caused by a dull rotary mower.

(Courtesy R. C. O’Knefski)

Fig. 35. Leaf blades and crowns killed where equipment was operated on dry, dormant grass. Symptoms appear when growth resumes after rain or watering. (Courtesy R. W. Smiley)

16

Beard, J. B. 2002. Turfgrass Management for Golf Courses. Ann Arbor Press, Chelsea, Mich.

Couch, H. B. 1995. Diseases of Turfgrasses. 3rd ed. Krieger Publishing, Malabar, Fla. Dernoeden, P. H. 2000. Creeping Bentgrass Management: Summer Stresses, Weeds, and Selected Maladies. Ann Arbor Press, Chelsea, Mich.

Elkeiy, T., and Ormrod, D. P. 1980. Response of turfgrass cultivars to ozone, sulfur dioxide, nitrogen dioxide, or their mixture. J. Am. Soc. Hortic. Sci. 105:664–668.

Hodges, C. F. 1992. Interaction of cyanobacteria and sulfate-­reducing bacteria in subsurface black-­layer formation in high-­sand content golf greens. Soil Biol. Biochem. 24:15–20.

Potter, D. A. 1998. Destructive Turfgrass Insects. Ann Arbor Press, Chelsea, Mich.

Richards, G. A., Mulchi, C. L., and Hall, J. R. 1980. Influence of plant maturity on the sensitivity of turfgrass species to ozone. J. Environ.

Qual. 9:49–53.

Smith, J. N. G. 2001. Managing black layer. Golf Course Management 69(12):59–62.

Turgeon, A. J. 2005. Turfgrass Management. 7th ed. Prentice Hall, Upper Saddle River, N.J. Vargas, J. M., Jr. 2005. Management of Turfgrass Diseases. 3rd ed.

John Wiley & Sons, Hoboken, N.J. Vittum, P. J., Villani, M. G., and Tashiro, H. 1999. Turfgrass Insects of the United States and Canada. 2nd ed. Cornell University Press, Ithaca, N.Y. Walker, C. 1971. Turf Culture. New Zealand Institute for Turf Culture, Palmerston North.