Medicago sativa and Melilotus indica
Alfalfa Stem Nematode (nematode – Ditylenchus dipsaci): The stem nematode has caused reduced yield in only a few Texas fields. Symptoms include shortened stems which become thickened and club-like. Crown buds infested with nematodes become swollen and distorted and will break off easily. The stem nematode is active only under cool, moist conditions. Plants grow normally during warm weather. Nematodes are introduced into fields by planting poor quality, nematode infected seed and by allowing run off water from infested fields to drain into other fields. A rotation with sorghum or small grains is also beneficial.
Dodder (parasitic plant – Cuscuta planiflora): This yellowish parasitic plant that has no leaves twists around the stems of host plants. Suckers or haustoria of dodder penetrate the host tissue and withdraw nutrients for their own use. It usually occurs in spots in the field. Seed produced by dodder are similar in size and color to those of alfalfa. This is how the plant usually becomes distributed from one area of the county to another. Seed fields should be screened carefully for occurrence of this parasitic plant and infested areas should be burned prior to harvest.
Mosaic (virus): Alfalfa mosaic is a widespread virus disease carried by insects, but it has never reached epidemic proportions. Light and dark streaks and blotches in the leaves are the main symptoms. Some stunting may occur. No special control is practiced in alfalfa.
Rust, Leaf Spots, and Stem Blight: Several species of fungi often cause leaf and stem spots on alfalfa but are not considered to be serious problems. Small, brown to black circular spots on the lower leaves are the most noticeable symptom. Some of the lower leaves may turn yellow and drop. The disease can develop rapidly when moist conditions exist.
Wilt Disease (bacterium – Corynebacterium insidiosum): Water conducting tissues in stem, crown, and roots appear brown to black. Plants are yellow and dwarfed. Stems are more numerous with bacterial wilt. Plants die after several months. The bacteria enters the plant roots and may live in the soil many years. Apply potash according to soil tests. Use crops other than legumes in the rotation for three to four years.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): This disease occurs in heavy alkaline soil but is not a problem in the High Plains, Panhandle areas, or the Brazos River bottom. Areas of dead plants occur in fields, usually in a circular pattern. Brownish, fuzzy strands of the fungus often appear on affected roots (See Illustration). The bark is dead and rotted on the tap root. When affected plants are pulled, they usually break off easily at or near the soil surface. It occurs in warm weather following rain or irrigation.
Downy Mildew (fungus – Peronospora trifoliorum): This disease is often seen in the spring when cool, moist weather exists. It may cause serious reductions in yield. The fungus, which causes new growth to turn yellow and new leaves to be twisted, infects that plant systemically for life and can remain in soil on old plants for several years. A white moldy growth is usually present on the underside of diseased leaves. Where severe, cut early to save as many leaves as possible. Prevention is possible by using clean, certified, and treated seed to establish new stands. Practice long rotations when the fungus is present.
Root and Crown Rot Complex (fungi and bacteria – Fusarium spp. Verticellium spp., Phytophthora spp., and others): This complex is one of our most troublesome alfalfa and clover problems. Stand decline is the most noticeable symptom (Note Illustration). Decline usually begins during the second year after planting and gradually becomes more severe. Early symptoms include yellowing and wilting of stem tips or entire shoots which eventually die. Plants are normally stunted and have an increased number of small, shortened stems and small leaves. Roots may be rotten or simply have a few discolored lesions. Reddish-brown streaks are usually evident when the outer bark is removed. Rotting may occur only at the crown area of the plant. Control is difficult. Use of tolerant alfalfa varieties is often the best method of control. Do not allow water to stand in a field for extended periods.
Seed Rot and Seedling Disease: Several soil fungi may attack seed as it germinates or may attack the young seedling after it has emerged from the ground. If the seedlings have emerged, the disease is characterized by a soft, water soaked area on the stem just below the soil line. Severe stand reduction can occur. This problem is most common when the soil is excessively wet. Chemical seed treatment may help reduce stand losses.
Production Practices which Reduce Alfalfa Diseases
- Prepare land before planting so the field has a uniform slope without low spots. A firm seed bed free of clods is necessary to get a good stand.
- Plant resistant varieties: Selecting the right variety is probably the most important decision to make in producing alfalfa. Resistant varieties offer the greatest protection from diseases as well as insects and nematodes. Always select a variety that is adapted to the area. Then select within the adapted varieties for the ones that show tolerance to diseases of concern in the area.
- Plant certified seedto insure getting pure seed of the desired variety.
- Treat seed with a fungicidesuch as thiram or captan to help prevent seed rots. Seed treatment often makes the difference between getting a good thick stand or a poor stand.
- Inoculate seed with nitrogen-fixing bacteriato increase yield and growth of alfalfa if planted on new land or on land that has not had alfalfa on it for several years.
- Plant in the fallto get the crop off to a better start. Fall planting is preferred because the seedlings usually have fewer weeds to compete with, and most weeds die with the first frost.
- Use the correct amount of fertilizerto obtain a healthy, vigorous, high yielding crop. Twenty pounds of nitrogen at planting time helps the seedlings get established while nodules are developing on their roots. Alfalfa is a high user of phosphorus and responds well to this element in most areas of the state. Apply 90 to 120 pounds of P2O5 depending upon a soil test.
- Irrigate properly.Alfalfa requires a lot of water to yield well. Alfalfa yields are higher, and it is less susceptible to root and crown rot diseases if light, frequent irrigations are used. Good drainage is needed.
- Cut during the early bloom stageto obtain high quality of hay. Early cutting helps prevent leaf diseases since tall, rank growth favors disease development. Early cutting also reduces losses of leaves when rust, down mildew, and leaf spot diseases are developing.
- Plow out thin stands of alfalfa when unprofitable. Alfalfa fields should be rotated to small grains, sorghum, or some other crops for at least two years before seeding back to alfalfa.