Reviewed October 1993
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Alfalfa is a very important forage crop in Missouri. Along with several serious insect problems (e.g. alfalfa weevil) alfalfa diseases are very important. Alfalfa diseases are caused by parasitic bacteria, fungi, viruses and nematodes. Diseases weaken plants so that growth is retarded, production is cut, stands are reduced, and they become vulnerable to winter injury. On the other hand, many nonparasitic disorders cause weakened plants, allowing the invasion of "opportunist" fungi and bacteria.
Annual disease losses of perennial legumes and grasses are difficult to assess because losses are usually additive and extremely variable depending on local situations. Foliar blights and root rots take their toll of plants and affect productivity of these crops as long as the stand is retained. Infections by crown and root rot fungi and viruses are not always immediately fatal. These pathogens frequently weaken plants so that they are unable to withstand adverse weather conditions such as drought or low winter temperatures.
Diseases cause serious losses in both yield and quality. Sometimes the damage occurs suddenly, but more often it is undetected until a disease causes serious injury. Diseases take from 10 to 20 percent toll annually without the grower's notice.
In order to curtail these losses, it is essential that the most common and destructive diseases be recognized. Good management practices will help greatly to control most of them. There are only a few chemical controls available, but those that are available are important (e.g. seed treatments).
| Disease | Symptoms | Remarks | Control |
|---|---|---|---|
| Root and crown diseases caused by fungi | |||
| Root and crown disease complex | Crown and root tissues damaged by heaving, desiccation or smothering. Plants may be killed or if less severly injured, may survive. | Winter injury can be the the cause of entry into root and crown tissues by parasitic fungi and bacteria. Therefore, a complex of non-parasitic and parasitic factors exist. | Use adapted varieties with resistant lines. Crop management designed to insure adequate storage of reserves-e.g. do not cut or pasture too late into fall. |
| Damping-off of seedlings Fusarium spp., Phytophthora magasperma, Macrophomina phaseolina |
Winter injury to roots and crown and parasitic invasion of injured tissues. | ||
| Several soil-inhabiting fungi, including Phytophthora megasperma, Fusarium spp. | Seed-rotting, pre-emergence blighting and post-emergence damping-off cause reduced stands in young seedlings. Soft rot of tissues and presence of sporangia and oospores in freshly rotted tissues are most certain symptoms-microscopic examination. | Widespread, especially in finer textured acid soils. | Use fully mature seed. Seed treatment under some conditions. Well-drained soils and well-prepared seed beds. Balanced fertility, including liming. |
| Phytophthora root rot Phytophthora megasperma |
Alfalfa plants rapidly wilt and die where water stands. Warm, dry weather after infection stunts plants, turns them yellow, and they slowly die. Infected crown and tap-root tissue turns soft, water-soaked yellow to tan. Infected plants pull up easily as crown breaks leaving root in soil. | Develops in heavy and poorly drained soils. Serious under cool, wet conditions. Varieties such as Agate, Apollo, Answer, Phytor, Trident, WL 220 have resistance | Avoid planting alfalfa in poorly drained soils. Treat seed with fungicide, such as metalazyl (Apron). Soil treatment with Ridomil may be desirable in some cases. |
| Rhizoctonia root rot crown rot, and foliage blight Rhizoctonia solani |
Root cankers are dark, sunken areas with brownish borders; usually circular; occur where young roots emerge from larger ones. Buds and shoots of crown are destroyed; plants weakened and invaded by secondary organisms. Foliage blight may occur with leaves and petioles showing water-soaked areas followed by collapse. | Associated with low flooded areas. Seasonal development-correlated with warm temperatures. Plant to plant advance may leave dead areas many feet in diameter. | Avoid planting alfalfa in poorly drained areas. Drainage operations. Crop rotation. Development of resistant varieties not well established. |
| Sclerotinia crown and root rot Sclerotinia trifoliorum |
Fall seeded alfalfa seedlings can be seriously affected. White cottony growth in the crown area. Stems and leaves wilt and turn yellow to grayish-green. Seedlings die. Black sclerotia develop on roots just below soil surface. | Sclerotinia Crown and Root Rot is potentially one of the most serious diseases of alfalfa. It has become seriously obvious since 1983. See MU publication G4558, Sclerotinia Crown and Stem Rot of Alfalfa. | Shift from fall to spring seeding in affected areas of state. Crop rotation 3-4 year intervals. Plow deep to bury sclerotia. No resistant varieties. |
| Foliar diseases caused by fungi | |||
| General | Foliar diseases are caused by several different fungi. Variations in the particular leaf symptoms will be found depending on organisms involved. Fungus organisms vary in response to environmental conditions, overwintering and survival, and relative susceptibility of host. | Foliar diseases usually do not kill alfalfa plants but cause losses in production by reducing quality and quantity of leaf material. Loss of vigor may set up plants to other diseases. | General control of Foliar Diseases:
|
| Common leaf spot Pseudopeziza medicaginis |
Leaf spot fungus starts on the lowest foliage in early spring and infection moves upward. Infected leaves often turn yellow and may fall, causing reduced hay quality and quantity. Leaf spots are circular, dark-brown about 1/16 inch in diameter. Small, dark brown disk (apothecium) in center of spots. | Occurs wherever alfalfa is grown. Seedling stands under thick cover crop may become heavily infected. Overwinters on fallen undecayed leaves. Fungus is not seedborne. | Heavily infected stands should be cut promptly in prebloom or bud stages, before leaves drop. Moderate resistance in varieties such as Vernal, Rambler or Teton. Widely grown varieties like Cody, Ranger, Saranac are only moderately to highly susceptible. |
| Yellow leaf blotch Letotrochila medicaginis (Pseudopeziza jonesii) |
Young lesions appear as yellow stripes and blotches parallel to leaf veins. As lesions enlarge, orange-yellow or brown colors appear. Similar blotches may appear on stems. Killed leaves have a tendency to remain attached to stems. | Disease is distributed less generally than the common leaf spot, but sometimes occurs along with common leaf spot and other leaf diseases. Found in rank, tall stands. | Cutting the crop before leaf drop is highly desirable as with common leaf spot. Burning leaves and stubble in early spring reduces spring inoculum. Most alfalfa varieties are moderately or highly susceptible. Same as other leaf spot diseases. |
| Stemphylium leaf spot (Pleospora herbarum) |
Limited almost exclusively to leaflets. Minute, light brown spots; similar to early symptoms of other leaf spots. Fully developed spots are almost oval or round and about 1/5 inch across with concentric rings, suggesting "target spots". | Fungus has wide host range among clovers and alfalfa. | Same as other leaf spot diseases. |
| Leaf rust Uromyces striatus f. medicaginis |
Reddish pustules on leaves, mostly on undersides of leaves. Seriously infected leaves may turn yellow and drop early. | Rust is not too damaging in Missouri because infections on alfalfa and clovers occur late in the season. Rust fungus overwinters in southern states and spreads northward in summer on south winds. | Same as other leaf spot diseases. Cherokee and Teton varieties have good resistance. |
| Downy mildew Peronospora trifoliorum |
Light green leaves especially at apex of stem and grayish-white mycelium on the surface of leaves. Internodal elongation is reduced, stems are smaller and leaflets are twisted and rolled in severe infections. | Widely distributed on alfalfa in temperate regions. Causes considerable damage to young plants resulting in reduced stands. Newly seeded alfalfa is most severly affected and will winterkill more easily. | Downy mildew is found in Missouri alfalfa fields, especially during wet springs. Some resistant varieties are available when needed. However, other diseases are more serious and varieties should be selected for their reaction. No management practices are practical. |
| Leptosphraerulina leaf spot or "Pepper spot:" Pseudoplea trifolii) |
Tiny, sunken black flecks develop on both surfaces of leaves; some form on young stems. Lesions enlarge to form sunken, gray- centered spots with reddish-brown margins. Tiny black dots may be sprinkled in centers in late fall and early spring. | This disease is prevalent and important, especially during prolonged moist weather. Attacks true clovers mainly, but alfalfa is also susceptible. Losses are hard to measure because "Lepto" occurs in combination with other leaf diseases. | Crop rotation and sanitation will help to keep infections low. Varieties differ in resistance. |
| Stem diseases (black stem complex) caused by fungi | |||
| Spring black stem Ascochyta imperfecta) |
Dark brown to black lesions on stems and leaf stems (petioles). Girdling on young shoots causes death. Small, black fruiting structures (pycnidia) are numerous on old stems. Brown spots on leaves are small, irregular and run together to form blackened areas. Infected leaves turn yellow, wither or curl before dropping. | Prevalent in Missouri. Occurs during cold, wet conditions of early spring, early summer and late fall. Head of class as being due to poor sanitation. Overwinters on old stems or crop residues. Spores are spread by splashing and blowing rains. Also seedborne. | Destroy crop residues by plowing or burning. Crop rotation-fungus persists two years. Early cutting of hay. Seed treatment if infected seed is used in areas free from the disease. No high resistance. Atlantic, Lahontan, Ranger are very susceptible. Buffalo, Rambler, Vernal are moderately resistant. |
| Summer black stem and leaf spot Cerospora zebrina) |
Dark reddish-brown to nearly black sunken areas on stem and petioles. Disease may girdle, blacken and kill new shoots and stunt plants. Leaf spots are small, gray-brown to black and rounded to irregular. Spots may run together to form large black blotches with yellow margins. Later leaves turn yellow or brown and drop early. | Occurs in association with spring black stem, but may be later. Fungus attacks upper parts of stems-not stem bases as in spring black stem. May be severe in second year's growth that is cut or grazed. Affects red clover and sweet clover also. | Same as for Spring black. Early cutting that is consistent with making high quality hay. |
| Anthracnose Colletotrichum trifolii |
Causes large, sunken elliptical stem lesions near soil line. Girding of stems in summer and fall. Also causes crown rot, with blue-black discoloration of tissues. | Fungus survives in infected stems and crowns in the field. Usually not as serious as spring and summer black stem diseases, but can enter into stem disease complex. | Anthracnose-resistant varieties include: Arc, Olympic, Riley, Saranac AR, Victor. |
| Wilt diseases caused by fungi | |||
| Fusarium wilt (Fusarium oxysporum f. medicaginis) |
Yellow, wilted and dead shoots in thinned stand. Dark red-brown streaks in vascular tissues of taproots. | Difficult to assess since many of the symptoms are characteristic of other wilt diseases. Diagnostic lab assessment is needed. | Adequate fertility permits alfalfa to escape serious Fusarium wilt injury. |
| Verticillium wilt (Verticillium alboatrum) |
Easily confused with Fusarium wilt or bacterial wilt and also may be mistaken for other root rot and crown rot diseases. Wilting of upper leaves followed by whole shoots wilting, turning pale, then yellowish to whitish and death. Xylem tissue is brown colored. | This disease has become prevalent in several northern states (e.g. Michigan, Minnesota, Wisconsin). So far, it has not been positively diagnosed in Missouri. But that does not mean it is not here. It is spread by air-borne spores or seed or infected plant parts. | Most U.S. varieties are very susceptible. Report and refer any suspicious symptoms to State Extension Diagnostic Lab. |
| Bacterial diseases | |||
| Bacterial wilt Corynebacterium insidiosum |
Alfalfa plants are dwarfed and reduced in vigor. Leaves turn yellow and bleached. Leaflets are smaller and thicker prior to loss of green color. Stems are smaller and more numerous in early stages. Plants die in late summer. Taproot shows pale brown discoloration of outer woody tissue when "bark" of taproot is peeled back. | Has been one of the most serious diseases in the past. Wilt usually most severe in low, poorly drained areas. Disease shows up in stands of susceptible varieties three or more years old. Survival is in living or dead alfalfa tissues in the soil. It is not seedborne. Spread is by surface water, renovating tools, animals, mowers and infected hay. With advent of resistant varieties, this disease is not prevalent. | For long-term alfalfa, plant only adapted, wilt-resistant varieties-such as Vernal, Cody, Buffalo, or Ranger. Newly seeded areas: maintain high, balanced fertility; mow when alfalfa is dry; cut young stands before older diseased fields; rotate infected fields two or more years; seed only wilt-resistant varieties in fields that get surface drainage from infested areas. |
| Bacteria leaf spot Xanthomonas alfalfae |
Leaf infections start as small water-soaked spots, which enlarge rapidly and coalesce to form irregular-shaped lesions. Most serious as a seedling disease-stunting and death. | Easily missed or confused with other leaf diseases. Overwinter on alfalfa stubble and spread by wind and rain. Enters through wounds. | Early cutting, as with other leaf diseases reduces leaf losses. |
| Rhizobium bacteria | Yellowing or slow growth. Nodulation poor. | With any leguminous plant the factor of nodulation and accompanying rhizobial population must be considered in studies of root function and disfunction. Antagonism between Rhizobium strains may be involved. | Inoculate alfalfa seed with correct rhizobium inoculum when seeding, especially in fields that have not had alfalfa. Keep pH level in 6 to 7 optimal range. |
| Virus and virus-like diseases | |||
| Alfalfa mosaic (AMV) | Greenish-yellow spots (mottling) followed by extensive yellowing of leaflets. Crinkling and irregularity in leaflets. Plants may be dwarfed in succeeding years. Symptoms are most severe during cool spring weather and are "masked" during the summer. | Aphid and mechanical transmission. Does not cause large direct reductions of seed, forage or hay, but plants may be weakened and susceptible to other infections. | Control measures are not very satisfactory. Resistant varieties are best answer, but are not known. Where economical, spray to control insects before they feed on healthy plants. |
| Witches' broom | Numerous crown buds that form fine stems shorter than normal. Leaflets are small with chlorotic margins and crinkling. Flower buds are poorly developed. | Occurs in western North America. Leafhopper transmission. No evidence of seed transmission. Not prevalent in Missouri. | Resistance not well known, but some tolerance does exist in alfalfa. Serious only in local areas. |
| Aster yellows (mycoplasma disease) |
Plants show various degrees of stunting and excess branching. Flowers show chlorosis, malformation and sterility, generally remaining on the plant rather than shedding as in Witches' broom. | Occurs infrequently in local areas. Leafhopper transmission. Aster yellows is very common on several wild hosts, so transmission is possible if leafhopper populations are high. | Not serious because of infrequency of infection. |
| Nematode diseases | |||
| Stem nematode Ditylenchus dipsaci |
Infects alfalfa at any age. Injury is mainly in the crown and young buds and stem bases are affected. Infected buds thicken and are deformed and usually don't elongate into stems. As spring advances, they turn dark and rot off, appearing as a crown rot. | Not many reports or substantiations. Destructive only in areas of high moisture, such as flood irrigation. Restricted host range-alfalfa, sweet clover, white clover. Overwinter in inactive state in infected buds and in soil and rubbish about crowns of plants. | Avoid flooded areas with alfalfa seedings. Resistance is obtainable in varieties such as Lahontan, Nemastan, Talent, and Washoe, but they may not be as adapted to Missouri conditions as other standard varieties. |
| Root knot nematode Hoplolaimus |
Field symptoms are not very obvious-only reduced growth and vigor. Characteristic root galls, which are sometimes found on alfalfa in regions where root knot nematodes are abundant on other crops. | Is not reported as serious disease on alfalfa, but should not be ruled out because it is not readily identified. If alfalfa is found with pea- sized galls on roots, contact Nematology Laboratory, 108 Waters Hall, University of Missouri, 573-882-7333. Very little is known of these species in Missouri. | Avoid seeding alfalfa in areas of root knot infestation if problems are experienced. |
| Insect injuries resembling diseases or disorders | |||
| Leafhopper injury | Yellowing and dwarfing of alfalfa plants. Leaflets are yellow and frequently brown along the margins. Terminal buds and early flower parts appear grayish-green and greatly retarded in development. | Internodal elongation, plant vigor, crown bud development and root survival are direct or indirect effects of the injury. In Missouri second or third cuttings are usually damaged most. | Heavy leafhopper infestation may need insecticide treatment. Consult latest Missouri Insect Control Recommendations for control if needed. |
| Other alfalfa insects: Alfalfa weevil, clover leaf weevil, pea aphid, spotted alfalfa aphid, variegated cutworm, webworm grasshopper | Symptoms from insect damage will vary from stunting and discoloration, yellowing, loss of stand in young seedlings, to ragged leaves and skeletonizing. These symptoms may be confused with diseases caused by microorganisms or they may mask their symptoms. | High populations in insects may suggest specific control measures. | Consult latest Missouri Insect Control Recommendations from MU Extension Division for description and appropriate control measures. |
| Nonparasitic disorders | |||
| Cold and winter injury -- see under root and crown diseases | |||
| Mineral deficiencies | |||
| Boron | Dwarfed plants and yellow to bronze foliage. Short terminal internodes, death of terminal buds and yellow or red foliage. White spots may appear in some plants. | May be found in certain agricultural areas. | If deficiency is known, application of minor elements including boron will help correct it. |
| Copper, molybdenum | Yellowing or stunting. | Minor element problems found only in certain localities. | Minor element applications -- Consult Extension Agronomy-Soils, 214 Waters Hall, 573-882-2001, Soil test and proper application of fertilizer. |
| Potash | Leaflets show yellowing and yellow blotches. May show firing along edges. | Found in soils deficient in potassium. | Soil test and proper application of fertilizer. |
| Phosphorus | Purple discoloration in leaflets and stems, especially along midribs. Seen especially in new growth. | Found in deficient soils. | Soil test and proper application of fertilizer. |
| Parasitic plants | |||
| Dodder | Leafless and almost rootless shoots which encircle plants. Small suckers (haustoria) penetrate food-conducting tissues of host plant. | Seed is found in great abundance during summer and matures until frost. Overwinters as seed. | Sow dodder-free seed. Clean combines and other equipment. Do not spread infested manure. |
When in doubt about a field diagnosis of alfalfa (and other forage diseases) make inquiries to:
G4551, reviewed October 1993