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 Alfalfa in Oklahoma is subject to many different disease organisms. Even in the driest areas of the state, diseases attack alfalfa and reduce yield, stand longevity, and sometimes forage quality. A few diseases are restricted to areas with high rainfall. The most important alfalfa diseases statewide in Oklahoma include:
Diseases can be a limiting factor in alfalfa production. In both hay and seed production, diseases can cause serious losses in yield, quality, and stand persistence. Death of plants is sometimes quick and obvious, but more often plant-kill by diseases is not so obvious. Diseases can kill 5-10 percent of the plants in a stand each year. In many cases, alfalfa plants live for several years after becoming infected by diseases. This reduces the vigor and competitiveness of infected plants; thus, weeds are better able to compete. The combined stress of diseases, weeds, and insects decreases productivity and eventually results in the death of infected plants. Mortality of plants results from diseases caused by fungi, bacteria, nematodes, and viruses. The most effective means of controlling the first six diseases listed above is the use of multiple-pest resistant varieties. All varieties released in the last decade possess adequate resistance to these diseases for most conditions in Oklahoma. It is important to note that much of the alfalfa known as Oklahoma common or VNS (variety not stated) lacks resistance to any of these important diseases. Maintaining good drainage, harvesting in a timely fashion, and using adapted, multiple-pest resistant varieties combine to provide an excellent management approach to control these diseases. While research continues on the last six diseases listed above, varieties with effective resistance are lacking, and proper cultural practices (including crop rotation, site selection, and seed bed refinement) are the only control options available. More progress has been made in the development of disease resistance in alfalfa than from directly breeding for increased forage yield or persistence. Identifying pathogens that cause symptoms on plants can be difficult. Producers are encouraged to consult the County Extension Office or the Plant Disease and Insect Diagnostic Laboratory at OSU for assistance in determining the causal agents. The following discussion gives a brief description of diseases that have been most frequently found on alfalfa in Oklahoma in recent years. Additional information on diseases, including photographs, can be obtained through the Oklahoma Alfalfa Calendar at Oklahoma Alfalfa.
Leaf and Stem Diseases Leaf and stem diseases that commonly infect alfalfa cause losses by reducing plant vigor, yield, and hay quality (primarily by causing leaves to drop). The effects of these diseases can range from slight to severe, even to the point of killing enough plants so that an alfalfa stand is no longer profitable. Losses from individual leaf spot diseases are difficult to measure since they are seldom found separately.
Downy Mildew. (See Photo 54). Appears as light grayish-green areas on the leaves, giving the field a light green appearance. Other fungus leaf diseases of alfalfa generally start on lower leaves and progress up the plant, but downy mildew symptoms appear first on younger leaves near the top of the plant. Infected leaves may be somewhat curled and distorted. Under severe infection, entire stems may be thickened and shortened. In high humidity, the lower surface of infected leaves shows patches of gray to slightly purplish downy growth, which represent the spore-producing of the fungus. Spores are disseminated by wind and splashing rain. Optimum conditions for downy mildew infections exist with near 100 percent relative humidity and temperatures between 50o and 65o F. In Oklahoma, this disease is most serious in the spring during the growth of the first crop. Newly seeded alfalfa is most severely affected. Susceptible seedlings may be killed or weakened so that they cannot survive any environmental stresses, such as a drought period. When downy mildew becomes severe, the alfalfa should be cut to stop disease development.
Lepto leaf spot. (See Photo 55.) Also called pepperspot. Can be found in alfalfa fields at almost any time in the growing season, but it is more prevalent during cool, wet weather. This is the first leaf spot to develop on regrowth after cutting. The leaf spots are variable from small (pinpoint) to large (1/8 inch) diameter spots with light tan centers and dark margins surrounded by a yellow halo. When conditions favoring infection and disease development coincide with rapid regrowth, lesions appear as large light tan to almost white areas that coalesce and kill entire leaves. Early cutting of hay is of little value in preventing this disease, since it builds up soon after harvest and older leaves become more resistant.
Spring black stem. (See Photo 56.) Can infect all aboveground parts of alfalfa, appearing as small black spots on lower leaves, petioles and stems. The infection may extend to the crown and upper roots. Blackened spots on stems are irregularly shaped. In a cool, wet spring, entire shoots are blackened, and stems become brittle and break. Cutting in the late bud to early bloom stage reduces losses from spring black stem because the disease builds up most rapidly after the growth is tall enough to shade lower leaves.
Summer black stem. (See Photo 57.) Recognized by loss of leaves and blackened stems. Also shows up as gray-brown leaf spots with irregular margins. The spots may be as large as 1/4 inch in diameter. Tissue around these spots soon turns yellow. One spot on a leaflet causes it to yellow and drop within a few days. Summer black stem generally appears after the first cutting of hay. The fungus develops most rapidly at temperatures of 80° to 90°F and near 100 percent humidity. Cutting the hay crop at bud stage is consistent with making high-quality hay and reduces losses from summer black stem.
Root and crown rots are usually caused by a number of common soil-borne fungi. These fungi weaken infected plants and reduce forage yield and stand longevity. Phytophthora root rot. (See Photo 58.) Causes damping-off of alfalfa seedlings and root rot in older plants. Seedlings are often killed within a few weeks after emergence or in spring after fall planting. Entire stands are sometimes killed. Young plants rapidly turn yellow or red, then wilt, and die. Infected taproots have brown to black lesions, and when severe, the lower end of the taproot is black and rotted off 1-6 inches below the soil surface. Phytophthora root rot occurs in wet, poorly drained soils during extended periods of rainfall or excessive irrigation. It can be detected by digging surviving plants in areas where stands have been thinned. If the soil dries out during disease development, some plants may recover by growing new lateral roots above the rotted area, but the resulting shallow root system limits drought tolerance and reduces yield. This is the most important alfalfa disease in Oklahoma. Essentially all soils have the fungus that causes this disease. The fungus can be dormant in the soil for many years and become active when the soil remains saturated for several days and the soil temperature is above 60°F. When the soil temperature is below 60°F, alfalfa can tolerate extended periods of flooding. All varieties for Oklahoma released in the last 20 years have adequate levels of resistance to this disease. Other than resistant varieties, the only form of control is assuring good internal and surface drainage before stand establishment. In poorly drained sites, Phytophthora root rot and other diseases
can kill most alfalfa plants after 1-2 years. Weedy grasses frequently invade these wet areas,
as there are not enough alfalfa plants to dominate the area.
Sowing white clover (Ladino) in these low areas can reduce the
weed infestation; however,
white clover dries more slowly in the windrow than alfalfa. Aphanomyces root rot. (See Photo 59.) An important disease of wet soils. It stunts and kills seedlings rapidly and causes a chronic root disease in established plants. Infected seedlings develop yellow cotyledons followed by chlorosis of other leaflets. Roots and stems initially appear gray and water soaked, and then turn light to dark brown. Seedlings become stunted but remain upright. Aphanomyces reduces root mass on established plants. Nodules are frequently absent or decayed. Infected plants exhibit symptoms similar to nitrogen deficiency and are slow to regrow following winter dormancy or harvest. For best results, select varieties with high levels of resistance to both Aphanomyces and Phytophthora root rot. Damping off. A germination and seedling disease. It is associated with less-than-ideal germination conditions, usually prolonged wet and cool periods. During germination, seeds may be infected and contents reduced to a brown gelatinous mass in the seed coat, or the radicle and cotyledons may become brown and soft after emergence. Pythium spp. and Rhizoctonia solani are normally the causal agents for damping off. Phytophthora and Aphanomyces may cause somewhat similar symptoms. Seed protectant fungicides are usually effective in reducing the degree of damping off. Planting in conditions that are conductive to rapid growth of seedlings will reduce seedling loss. Damping off is more likely to be a problem with spring planting or planting after early October.
Fusarium root and crown rots. (See Photo 60.) Caused by several species of the Fusarium fungus. External symptoms are first evident when the leaves curl at the edges, then wilt. When the taproot is cut lengthwise, it is discolored light brown to black. Damage varies from irregular brown rotted areas to complete disintegration of the root and crown. Such rots usually occur after injury from freezing, harvesting, or insect feeding. The disease generally progresses slowly and becomes most noticeable following periods of stress caused by insufficient moisture, severe heat, or mismanagement. All adapted multiple-pest resistant varieties have good levels of resistance, which is the only known control for this disease.
Texas root rot. (See Photo 61.) Also known as Phymatotrichum root rot and cotton root rot. In Oklahoma, this disease occurs only along the southern border. The Phymatotrichum fungus does not survive cold winter temperatures. This root rot causes rapid death of plants, often after excellent growth. The disease kills plants in somewhat circular areas within fields ranging from a few square yards to an acre or more in size. Death occurs within a few days of the first wilting of the plants. The whole root system of affected plants decays, and the plants can be pulled from the soil with little effort. Affected plant roots show a covering of fine brownish strands of the fungus. These strands are called rhizomorphs and are a distinguishing characteristic of the disease. Cereal crops and sorghum are resistant to Phymatotrichum root rot, but because this organism persists for long periods, traditional 3-6 year crop rotation of alfalfa and cereals is ineffective as a control measure. There are no known resistant varieties to Texas root rot.
Bacterial wilt. Symptoms begin to appear in the second and third year and may cause serious stand losses in 3- to 5-year old stands. Affected plants turn yellow, become stunted, and in early stages of disease, are scattered throughout the stand. Severely infected plants are stunted with many spindly stems and small, distorted leaves. Diseased plants are most evident in regrowth after clipping. Cross sections of the taproot show a ring of yellowish brown discoloration near the outer edge. All adapted multiple-pest resistant varieties are resistant to this disease.
Verticillium wilt. Can reduce yields up to 50 percent beginning the second harvest year and can severely shorten stand life. Early symptoms include v-shaped yellowing on leaflet tips, sometimes with leaflets rolling along their length. The disease progresses until all leaves are dead. Initially, not all stems of a plant are affected. The disease slowly invades the crown, and the plant dies over a period of months. Root vascular tissues may or may not show internal browning. Many alfalfa varieties are resistant to this disease. The following measures minimize the chances of introducing this fungus and spreading the disease between and within fields:
Sclerotinia crown and stem rot. Caused by the same fungus that causes Sclerotinia blight in peanuts. Consequently, alfalfa should not follow peanuts when Sclerotinia blight has been a problem. The Sclerotinia fungus can kill alfalfa seedlings rapidly. Symptoms first appear in the fall as small brown spots on the leaves and stems. Then the plant tops wilt and die. In early spring, crowns of infected plants become soft with a brown white fluffy mass of fungus mycelium. Hard, black fruiting bodies (sclerotia) form in this fungus mass and may adhere to the surface or be embedded in the stem or crown area. These hard, black fruiting bodies are the survival stage of the fungus. Sclerotinia can survive in the soil for up to four years. This disease in not known to occur in Oklahoma alfalfa. In areas of the nation where this fungus is severe, fall-planted stands are frequently devastated; consequently, late summer and fall stand establishment is avoided. The disease is rarely important in spring-planted stands.
Viruses A number of viral diseases of alfalfa have been described, but relatively little is known about their distribution and importance in alfalfa, with the exceptions of alfalfa mosaic virus, alfalfa enation virus, and lucerne transient streak virus. Often, viruses are carried by aphids, leafhoppers, and other plant-sucking insects. There is no effective control of viruses, but minimizing the various plant-sucking insects by scouting, timely treatment, and using resistant varieties may offer a degree of control.
Nematodes Plant-parasitic nematodes are microscopic roundworms that cannot be seen with the unaided eye. Stem nematodes, root lesion nematodes, and rootknot nematodes are the most damaging plant parasitic nematodes found in Oklahoma alfalfa fields. Their importance is a result of their pathogenic effects on alfalfa and their ability to become involved with other pathogens in disease complexes, thereby increasing disease severity. The only way to know for certain if nematodes are present is to collect soil and roots, or in the case of stem nematodes, above-ground plant parts, and send a sample to a laboratory equipped for extracting and identifying nematodes. For details on sampling for nematode analysis, consult the County Extension Office or the Plant Disease and Insect Diagnostic Laboratory at OSU.
Alfalfa stem nematode. One of the few nematodes that feed on plant foliage, it can cause severe stunting of alfalfa. Infected plants have a bushy appearance and usually possess thickened stems and shortened internodes (See Photo 62). Leaves of infected plants often appear crinkled. High populations of stem nematodes cause stunting and can kill alfalfa plants. A stand of alfalfa may decline quite rapidly after stem nematodes become established, sometimes reducing alfalfa production enough to be unprofitable one year after infection is detected. Symptoms of alfalfa stem nematodes are normally noticed in the second or third production year. The first symptoms may be limited to small areas during late March or early April. The symptoms may disappear as temperatures rise, but they will return in following years over larger areas until most of the alfalfa plants in the field have symptoms of damage. Stand thinning accelerates when the stem nematode is present. Resistant varieties are available but do not seem to be effective in Oklahoma. The best control for stem nematodes is to prevent their spread by cleaning any remaining alfalfa hay, plant parts, and soil from haying equipment before moving from an infested area. The nematodes are carried in plant debris on equipment from one field to another. Rotation to crops such as small grains, sorghum or corn for 2-3 years will reduce alfalfa stem nematode and rootknot nematode populations; however, crop rotation is an ineffective control of root lesion nematodes because of their wide host range. Root lesion nematodes. Widespread in their distribution, occurring frequently throughout temperate regions, they attack a wide range of crop and weed plants. High infestations of root lesion nematodes can reduce forage yields, decrease cold tolerance, and increase infections by soil-inhabiting fungi. Damage caused to alfalfa plants is enhanced by root lesion nematodes predisposing roots to soil microorganisms. Aboveground symptoms of root lesion nematodes are difficult to assess since other pathogens may produce similar symptoms. No visual symptom develops when nematode numbers are low, but when numbers are high, even when plant growing conditions are ideal, infected plants become stunted. Feeding by root lesion nematodes generally causes dark lesions, overall browning of roots, and reduced growth. Rootknot nematodes. Probably the most widely disseminated plant parasitic nematodes in the world, they rank high in economic importance as plant pathogens. Rootknot nematodes may build up large populations on alfalfa and are a major alfalfa production problem in many states. In Oklahoma, however, they appear to be less frequently encountered on alfalfa and are probably less important than stem and root lesion nematodes. Like the root lesion nematode, root-knot nematodes may be involved in disease complexes with other alfalfa pathogens. Alfalfa plants infected by rootknot nematodes may become stunted, and the stand may be reduced. Infected roots branch excessively and have small galls. Rootknot galls are swelling of the tissue of the root itself, frequently involving the total diameter of the root, and should not be confused with Rhizobium bacterial nodules which usually appear to be appendages attached to the root.
MAMMALIAN PESTS Mammalian pests that occasional reduce alfalfa production or persistence and interfere with normal operations include rabbits, moles, gophers, voles, mice, rats, and deer. Cultural practices, combined with baiting, are the only practical control options.
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Alfalfa Production Guide for the Southern Great
Plains, 2001
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