Compendium of
Alfalfa Diseases and Pests THIRD EDITION
Introduction Alfalfa (Medicago sativa L.) is the most important forage legume species in the world. In the United States, alfalfa is grown on more than 9 million hectares, with an annual crop value of more than $8 billion. Worldwide production is approximately 32 million hectares and is increasing with growing market demand for dairy products. Alfalfa is known as the “queen of the forages” owing to its high feed value for livestock. Most of the crop is grown for hay, but alfalfa is also grown for other products including haylage, cubes, and meal, and is widely used as a component in pastures and grazing land. Approximately 7% of the seed produced in the United States is used for growing alfalfa sprouts for human consumption. Because of its high biomass production, deep taproot, and nitrogen-fixing capabilities, alfalfa is ideal for use as a cover crop as well as for site reclamation and soil rehabilitation. Nontraditional uses of alfalfa in bioenergy, as a source of protein and phytonutrients for human foods, and as an industrial feedstock in biorefining have been explored. Alfalfa is amenable to genetic modification for cultivar improvement and for the introduction of native and novel traits.
History of Alfalfa Alfalfa is indigenous to the Caucasus region of Eurasia, northwestern Iran, and northeastern Turkey. It belongs to a species complex that includes diploid and tetraploid interfertile subspecies. There is evidence that alfalfa was used as a forage crop as early as 1400–1200 b.c. Seed was taken by traders, explorers, and invading armies to many regions throughout the world for the purpose of planting and growing forage for horses. In many areas, alfalfa survived and became naturalized, sometimes interbreeding with native Medicago species in the region. The Spanish and Portuguese took alfalfa to the New World in the 1500s to grow feed for their horses. The name alfalfa is believed to be derived from a Persian word meaning “horse fodder” or the Arabic and Kashmiri words for horse power. Alfalfa is known as lucerne in the United Kingdom, Australia, South Africa, and New Zealand; erba medica in Italy, meaning “medical herb”; luzerne in France; and lucerne grass in South Asian English. Introductions into eastern North America appear to have occurred during the 1700s, but these introductions were not highly successful, possibly as a result of the acid soils and humid climate throughout much of the region. In the mid 1800s, alfalfa (termed “Chilean clover”) was introduced into California, where it thrived. Acreage of alfalfa gradually moved eastward, and with the advent of winter-hardy selections, gradually became the predominant forage crop in the eastern United States. Introduction into Australia and New Zealand occurred in the early 1800s and introduction into South Africa occurred in the mid 1800s. Annual Medicago species are also cultivated for forage and in pastures.
The Alfalfa Plant Modern cultivated alfalfa is a perennial, open-pollinated, highly heterozygous and heterogeneous, and autotetraploid plant. A population of alfalfa plants, although identified by a particular set of phenotypic characteristics, is nevertheless a diverse collection of genotypes. Most cultivated alfalfa is purple-flowered M. sativa subsp. sativa, although pure stands of yellow-flowered M. sativa subsp. falcata are also grown because of its cold hardiness. Because alfalfa seeds are small and alfalfa seedlings are rather fragile (Fig. 1A and B) and vulnerable to several diseases, they often suffer high mortality. Seeding rates are chosen to compensate, and seed treatments are used to reduce some disease problems. Well-drained soil helps minimize seedling diseases because the motile spores of some pathogens require water for movement to roots and because alfalfa roots are predisposed to infection in a water-saturated soil environment. Alfalfa has a deep taproot, extending 1 to 2 m per year into the soil profile. This feature gives alfalfa the ability to obtain water and nutrients from far below the soil surface and makes alfalfa one of the most drought-tolerant of the major crops. However, because of its dependence on a single taproot for water and nutrients, an alfalfa plant is vulnerable to root- rotting pathogens such a Phytophthora medicaginis and Phoma sclerotioides, which may girdle the taproot just below the soil surface. Vegetative buds originate from the crown, a meristematic region that develops just above the taproot (Fig. 1C and D). Stems arising from these buds grow rapidly, often attaining a height of 60 cm or more before flower buds begin to form. As stems grow, new vegetative buds form on the crown and give rise to a new crop of shoots when the previous growth is removed by cutting or grazing. Depending on climatic conditions, this process can repeat itself several times during the growing season and this determines the number of harvests possible. In colder regions of North America, only two harvests may be possible, while in warmer regions of the midwestern to the southern United States, three to five harvests are common. In the warm climate of the southwestern United States under irrigated conditions, “nondormant” alfalfa may continue to regrow year round and provide 10 or more harvests per year. Alfalfa is a “nonreseeding” legume with essentially no capability for vegetative proliferation, e.g., by means of stolons or rhizomes. Although seed may be abundantly produced in an uncut field, relatively few seeds germinate and virtually no seedlings develop into mature plants. This is due primarily to the phenomenon termed “autotoxicity.” Alfalfa plants produce water-soluble toxins, concentrated mainly in leaves, that impair development of the seedling taproot and results in stunted, weak plants that are more susceptible to stress. A waiting period is needed to allow toxins to degrade before replanting a field to alfalfa. Thus, all plants in a field develop from seeds that germinate shortly after planting and no further increase in 1
Fig. 1. Growth stages of an alfalfa plant. A, Seedling; B, young plant with first trifoliolate leaf; C, three plants at the initiation of crown development; and D, crown of a mature plant. (Courtesy W. Hoffman)
plant number will take place during the life of the stand. Lethal diseases such as Fusarium wilt and anthracnose kill individual plants. As plants die, the stand thins to the point where it is no longer productive and must be replaced. In the midwestern United States, the life of an alfalfa stand is typically 3–4 years, although it is possible to maintain stands for longer periods.
Symbiotic Interactions Alfalfa plants participate in two important beneficial, symbiotic interactions with soil microorganisms. The interaction of alfalfa with the nitrogen-fixing bacterium Sinorhizobium mel iloti has been investigated in detail and results in formation of a new organ—a cylindrical, pinkish root nodule (Fig. 2)—in which modified root cells are colonized by the bacteria. The bacterial enzyme nitrogenase converts atmospheric nitrogen (N2) to ammonia, which is then assimilated by the alfalfa plant into amino acids. This process of biological N2 fixation is a major means worldwide by which atmospheric nitrogen is added to crops and soils. Two or more years of alfalfa cultivation provides all of the nitrogen fertilizer needs of a subsequent grain crop. Alfalfa plants that cannot form an effective symbiotic interaction are useful for remediating soil and water with excess nitrogen. Commercially produced alfalfa seed is coated with a mixture of S. meliloti strains selected for maximum fixation potential. The second type of symbiosis, mycorrhizal fungal interactions with alfalfa, has been less well studied but is no less important for optimal plant performance. Plants with mycorrhizae (fungus- roots) typically exhibit improved growth and yield when compared with plants growing under similar conditions but lacking mycorrhizae. The principal benefit to the plant is 2
Fig. 2. Root nodules containing the symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti. (Courtesy B. Bucciarelli)
an increased supply of phosphorus or other immobile nutrients, such as copper or zinc. Phosphorus is taken up by hyphae outside the root, translocated to fungal haustoriumlike structures called arbuscules within the root, and ultimately released to the plant. Numerous secondary effects result from increased phosphorus inflow, including increased resistance to water deficit, improved resistance to some diseases, and enhanced nodulation and nitrogen fixation. The alfalfa root morphology does not change conspicuously as a result of colonization by arbuscular mycorrhizal fungi. However, mycorrhizal fungi occupy a large volume of the cortex of small lateral roots (Fig. 3).
Fig. 3. Colonization of a root by a symbiotic mycorrhizal fungus. (Courtesy L. H. Rhodes)
Because the cortex is sloughed off as roots undergo secondary thickening, mycorrhizal fungi are not found in taproots or in the larger lateral roots. Arbuscular mycorrhizal fungi are ubiquitous in soil, and mycorrhizal associations are the rule rather than the exception. Some of the more common genera are Glomus, Sclerocystis, Gigaspora, Scutellospora, Acaulospora, and Entrophospora. These fungi are worldwide in distribution, with more than 200 species having been described. Little host specificity exists among the fungal symbionts. For example, Glomus mosseae forms mycorrhizae on alfalfa and other forage legumes as well as on many woody and herbaceous dicots, monocots, and even several woody gymnosperms.
Alfalfa Disease and Pest Resistance Breeding Most alfalfas grown in the early twentieth century were locally adapted selections such as “Kansas Common” with virtually no disease resistance. Early efforts to improve alfalfa were aimed at cold tolerance. “Grimm” alfalfa was a cold-tolerant selection adapted to the northern United States. Grimm and other cold-tolerant selections paved the way for introducing alfalfa into the northern and eastern United States. Since the 1930s, considerable effort has been made to incorporate disease and insect resistance into alfalfa. In 1940, F. R. Jones observed, “For a number of years bacterial wilt has been recognized as the most important disease of alfalfa in the United States.” In that same year, the bacterial wilt-resistant variety Ranger was released. Since then, hundreds of bacterial wilt-resistant varieties have been developed. Today, virtually all commercial varieties available within the United States have high levels of resistance to bacterial wilt, making it a disease that has all but disappeared from production fields. Resistance to several other diseases followed, and most varieties today have adequate levels of resistance to Fusarium wilt, anthracnose, Phytophthora root rot, stem nematode, Verticillium wilt, Aphanomyces root rot, and, as already noted, bacterial wilt. A few varieties have resistance to Sclerotinia crown and
stem rot, with the number likely to increase in the near future. Each year a list of currently available cultivars and their disease and insect resistance levels is published by the National Alfalfa and Forage Alliance. Largely because of the autotetraploid nature of alfalfa and its proclivity for cross-pollination, alfalfa breeding is done largely by phenotypic recurrent selection. For disease resistance breeding, this involves inoculation of an alfalfa population with a given pathogen and selection of surviving (or less severely damaged) individuals to be parent plants for the next cycle of selection. This process may be repeated for several cycles and for many different traits. The populations developed from this process are used as the parent populations for breeding commercial varieties (cultivars). Standard tests have been developed for many alfalfa diseases to aid in selection of resistant plants and for characterizing resistance in cultivars. Because alfalfa varieties are not genetically uniform like soybean or wheat varieties, they are often referred to as “synthetic” varieties. A synthetic alfalfa population is composed of both plants that are resistant and susceptible to a specific pathogen. The genetic diversity of a synthetic variety provides resistance with long-term stability and is thus one of the most valuable features of alfalfa. Unlike most other major crops, in which the genetic uniformity of varieties or hybrids promotes selection pressure for the emergence of new pathogen races, alfalfa, with its diverse genetic makeup, is rarely damaged by new biotypes. The pathgens of only three diseases—anthracnose, Aphanomyces root rot, and downy mildew—are known to have multiple races, and these races have either not been highly damaging or have been managed successfully through introduction of resistant varieties. As indicated above, alfalfa-breeding efforts have been aimed largely at developing cold tolerance, increasing yields, and improving pest and disease resistance. These characteristics were considered primarily within the context of hay- production systems. However, since the early 1970s, several additional traits have been the focus of breeders’ attention. Particularly noteworthy have been efforts to improve forage quality (nutritional value), increase tolerance to grazing, and most recently, incorporate tolerance to the herbicide glyphosate. Undoubtedly, the future will see continuing efforts to develop alfalfa with specialized traits, including the incorporation of specific genes to improve forage digestibility and reduce problems of bloat. Use of DNA markers for selecting germplasm with disease resistance and other traits is likely to increase in alfalfa-breeding programs.
Overview of Alfalfa Diseases Alfalfa diseases are caused by biotic (infectious) agents and abiotic (noninfectious) agents. Abiotic agents are primarily extremes of the environment such as flooding, drought, high and low temperatures, low or high soil pH, nutrient deficiency, or chemical toxicity. Biotic agents, called pathogens, include fungi, oomycetes, bacteria, viruses, phytoplasmas, nematodes, and parasitic plants. In order for an infectious disease to occur, there must be a pathogen, a susceptible plant, and an environment conducive to disease development. For example, if spores of a pathogenic fungus land on a susceptible plant but water is not available to facilitate spore germination, infection will not occur and the disease will not occur. Effects of disease on individual plants vary widely. Some diseases are lethal while others cause only mild stunting or leaf loss. For example, most fungal leaf spot and viral diseases rarely kill plants, while seedling diseases, vascular wilts, and crown and root rot diseases often result in plant death, sometimes within a matter of days. Diseases in which a single pathogen has the capability to kill plants have been referred to as 3
“acute” diseases. Examples of acute diseases in alfalfa include Fusarium wilt, anthracnose, Phytophthora root rot, and Sclerotinia crown and stem rot. Nonlethal diseases such as Leptosphaerulina leaf spot, alfalfa mosaic, and root-k not nematode have been termed “chronic” diseases. With chronic diseases the individual pathogens may not be capable of killing plants outright, but collectively they may impose an “accumulative stress load” on a plant that ultimately results in its death. A third category includes those problems known as “disease complexes.” For example, crown rot is a lethal disease thought to be the result of the combined action of several pathogens growing in the crown region of the plant. Fusarium spp., Rhizoctonia solani, Colletotrichum spp., Phoma spp., Pythium spp., and other pathogenic fungi, as well as Pseudomonas spp. and other bacteria can often be isolated from the rotted crowns of dead and dying plants. The relative importance and interactions of each of these pathogens is still unclear. Identification of the source of inoculum and its means of dissemination is often the key to understanding disease development. Infective propagules of alfalfa pathogens frequently reside in soil or in buried plant residue from the current or previous crop. Such soilborne pathogens may be present when the alfalfa crop is planted and may infect plant roots in close proximity. Alternatively, such propagules could be carried to plant roots by soil insects such as the clover root curculio, or, if close to the soil surface, could be splashed to aboveground plant parts. Some soilborne pathogens (e.g., Sclerotinia trifo liorum) survive in the soil in a dormant state. Under favorable environmental conditions, these fungal pathogens produce specialized fruiting structures capable of releasing spores into the air. With S. trifoliorum, a single fruiting body is capable of producing millions of airborne spores. Spores land on susceptible alfalfa plants, germinate, and penetrate leaf and stem tissues. Because the crown and lower stems (stubble) are not removed in the harvesting process, many pathogens survive and multiply on these tissues. Phoma medicaginis, the cause of spring black stem and leaf spot, overwinters as pycnidia in the stubble where it produces spores (conidia) adapted to water-splash dissemination. When new growth emerges in the spring, shoots grow from the crown up through the stubble and can be readily infected by water-dispersed spores. Inoculum of some pathogens is spread from one plant to another on harvesting equipment. Spores of the vascular wilt pathogens Fusarium and Verticillium spp., for example, are contained within the xylem tissue of the stems and contaminate cutter bars during harvest. As harvest equipment moves through the field, spores may be deposited directly on the cut surfaces of healthy plants where they can germinate and grow directly into the exposed xylem. Most alfalfa viruses are transmitted by insects, particularly aphids. Alfalfa mosaic virus (AMV), for example, is known to be carried by at least 14 aphid species.
Disease Identification Even with the best crop management, some disease do occur. Knowing which diseases are likely to occur in specific areas is key to selection of cultivars, and disease diagnosis is critical to determine the appropriate management strategies. This compendium can be used as a guide for disease diagnosis. The selected references should be consulted for additional information on pathogen identification and disease management. Some diseases have very similar symptoms, such as the crown and root rot diseases, and a specific diagnosis from symptoms alone is not possible. Isolation of the pathogen in pure culture is needed to confirm pathogen identity. Additionally, DNA-based identification tests are increasingly used for pathogen identification from pure cultures or from infected plant materials. Commercial serological tests are available for many plant viruses. 4
Disease Management Alfalfa disease losses may be reduced substantially by using sound crop production practices. Achieving and maintaining proper soil pH (between 6.5 and 8.0), adequate soil fertility, and good drainage are essential. Proper harvest intervals also are critical to the maintenance of a vigorous alfalfa stand. These and other practices that alleviate plant stress and improve vigor may allow plants to be more resistant to disease agents or to withstand the effects of disease once they become infected. More importantly, good growing conditions allow surrounding uninfected plants to achieve their maximum potential and to compensate for stand losses caused by disease. It should be recognized, however, that many pathogens are as likely (or more likely) to attack healthy plants in well-managed stands as those that are under stress resulting from adverse environmental conditions or mismanagement. Field selection. In addition to pH and fertility considerations, alfalfa fields should be well drained. Areas subject to temporary flooding should be avoided or their soil drainage should be improved, because many seedling diseases and root and crown rot diseases are favored by wet soil conditions. Crop rotation. Pathogens often build up to high populations during the life of an alfalfa stand. When alfalfa follows alfalfa in a rotation, these organisms may attack the new crop as soon as it is planted. Further, autotoxic chemicals from alfalfa plants in the previous planting may cause injury to germinating seed and developing seedlings in the new planting. When possible, alfalfa should follow corn or a small grain in the rotation because these crops have relatively few pathogens in common with alfalfa and are thus less likely to provide a source of pathogen inoculum for the new alfalfa crop. Timely harvest. Leaf spot diseases reduce yield and quality by causing leaf drop. Harvesting on or before schedule can reduce the amount of leaf loss and also minimize the buildup of inoculum. Minimizing traffic over the field. Soil compaction and direct injury to alfalfa crowns occur each time a trip is made over the field. The damage from disease organisms is frequently increased in heavily traveled areas. Movement of hay, manure, and infested machinery. Pathogens may be spread from an infested field to a healthy one by transport of harvesting equipment, hay, or manure. Harvest healthy fields before harvesting fields that are obviously diseased. If possible, harvesting equipment should be thoroughly cleaned prior to entering uninfested fields. Variety selection. Planting disease-resistant alfalfa varieties is one of the most important aspects of disease management. Virtually all new varieties have field-relevant levels of resistance to the most damaging diseases. However, because an alfalfa variety is actually a mixture of genetically different plants, not all plants within the variety carry resistance to a particular disease even though the variety may be rated as “resistant.” For an alfalfa variety, the measure of disease resistance is expressed as the percent of plants exhibiting resistance to a particular pathogen based on a standard test. The scale, with corresponding designations for the numerical percentages, is shown in Table 1. For example, a variety listed as “resistant”
to Phytophthora root rot may have only 31–50% of the plants genetically resistant to the pathogen; the remaining 50–69% of the plants in the variety lack genetic resistance and would be classified as susceptible. Nevertheless, the advantages of varieties with even moderate levels of resistance are obvious when compared with varieties classified as susceptible to a given disease. Chemical control. Historically, few pesticides have been available for the control of alfalfa diseases. Seed treatment formulations of metalaxyl or mefanoxam (Apron) are effective in controlling seedling diseases caused by Pythium and Phytophthora spp. Seed is often treated commercially with these chemicals prior to being sold. Cupric hydroxide and other copper-based fungicides have long been available for controlling foliar diseases such as Leptosphaerulina leaf spot and summer black stem and leaf spot; however, they have not been widely used throughout the United States. Recently, several new disease control products have been labeled for use in alfalfa, including a number of products with active ingredients that are considered “natural chemicals,” such as hydrogen peroxide, as well as biocontrol agents such as Bacillus subtilis. Other fungicides recently labeled on alfalfa include the strobilurins azoxystrobin and pyraclostrobin, which have been effective on certain diseases in other crops. It remains to be determined how all of these products fit into modern alfalfa disease management programs.
Selected References
Beuselinck, P. R., Bouton, J. H., Lamp, W. O., Matches, A. G., McCaslin, M. H., Nelson, C. J., Rhodes, L. H., Sheaffer, C. C., and Volenec, J. J. 1994. Improving legume persistence in forage crop systems. J. Prod. Agric. 7:311-322. Bolton, J. L. 1962. Alfalfa: Botany, Cultivation, and Utilization. Interscience, New York. Bouton, J. 2007. The economic benefits of forage improvement in the United States. Euphytica 154:263-270. Dilworth, M. J., James, E. K., Sprent, J. I., and Newton, W. E., eds. 2007. Nitrogen-fixing Leguminous Symbioses. Springer, Dordrecht, The Netherlands. Hanson, A. A., Barnes, D. K., and Hill, R. R., Jr., eds. 1988. Alfalfa and Alfalfa Improvement. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, WI. Miller, S. A., Beed, F. D., and Lapaire Harmon, C. 2009. Plant disease diagnostic capabilities and networks. Annu. Rev. Phytopathol. 47:15-38. National Alfalfa and Forage Alliance. 2012. Winter survival, fall dormancy and pest resistance ratings for alfalfa varieties. http://www. alfalfa.org/varietyLeaflet.php North American Alfalfa Improvement Confererence. 2004. Standard Tests to Characterize Pest Resistance in Alfalfa Cultivars, Third Edition (Amended 2004). http://www.naaic.org/stdtests Russelle, M. P. 2001. Alfalfa. Amer. Sci. 89:252-261. Tesfaye, M., Samac, D. A., and Lamb, J. F. S. 2008. Alfalfa. Pages 199-210 in: Compendium of Transgenic Crop Plants: Transgenic Legume Grains and Forages. C. Kole and T. C. Hall, eds. Blackwell, Chichester, U.K.
5