Plant Disease Control No. 12
Revised February 1998
Verticillium Wilt of Alfalfa
Verticillium wilt is the most serious disease problem of alfalfa in the north-temperate areas of the United States, Canada, and Europe. It was discovered in the Pacific Northwest in 1967 and was first found in Northern Utah in 1985. It has subsequently been found in most counties in Utah. The widespread occurrence of the disease in the state suggests that it may have been present, but undetected, for several years.
Verticillium wilt kills plants and severely reduces expected stand life. The disease is usually not detected until after the third year. The Disease can spread rapidly and yields can be reduced as much as 50% by the third or fourth year. The life of the stand can be reduced from the normal 6 or 7 years to 3 or 4 years. Other cultural or disease problems can be confused with Verticillium wilt. Check with your county agent if you suspect the disease. Positive identification may require culturing the fungus from infected plants.
SYMPTOMS
The first evidence of the disease is a yellowing of the young leaflets near the top of the affected plant. Infected leaflets may develop a "V" shaped, yellow or brown area at the tips. The most diagnostic symptom of Verticillium wilt occurs when the leaves of the plant turn a bleached tan to pink color while the stem remains upright and green.
The symptoms most often appear during the 2 weeks prior to harvest but can be present anytime of the year. When temperatures exceed 90 F (32 C), the symptoms may become less conspicuous or nonexistent.
CAUSES
Verticillium wilt of alfalfa is caused by a specific strain of the fungus, Verticillium albo-atrum. The strains of Verticillium that attack tomato and potato will not infect alfalfa. The fungus is present in infected alfalfa seed and chaff, but is primarily spread by hay from infected fields or on equipment.
The pathogen is most aggressive under cool temperatures (65 F) and moist conditions. Spores are produced on diseased plants and spread by wind or machinery. The fungus can survive in dry hay for at least 1 year. It does not survive in the soil for more than a year in the absence of a suitable host.
CONTROL
1. Prevent introduction into areas free of the disease.
- Seed should be of high quality and preferably certified. There is currently no certification for freedom from Verticillium.
- Do not feed hay from affected areas on alfalfa fields because the fungus survives for at least one year in dry hay.
- Cut young fields first to prevent spreading the disease on equipment from old established fields to new fields. Clean equipment thoroughly after cutting an infested field.
2. Plant varieties that are resistant to Verticillium as well as other diseases present in your area.
- Resistance to Verticillium has been incorporated into many new varieties. A list of recommended varieties can be obtained from your county agent. This is the most effective way to control the disease.
3. A rotation out of alfalfa for at least one year with a non-host crop such as wheat, barley, corn, or grass pasture will reduce the soil inoculum. The pathogen survives in the soil for less than one year but will survive longer on some weeds or volunteer alfalfa plants.
4. Eradicate infected plants if present in small numbers or limited areas.
The disease spreads from localized spots and can be kept in check by thoroughly inspecting fields. Most infections show up first near entry points. Destroy infected plants and any adjacent plants. The affected areas should be clearly marked and inspected periodically to insure eradication.
Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
Listing of commercial products implies no endorsement by the authors or the Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Persons using such products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its prope
use. Always read and follow the label.