Cause Sclerotinia trifoliorum or S. sclerotiorum, which are soilborne fungi that can survive indefinitely as sclerotia in soil. They can attack stems and crowns in wet, cool springs. Damage is most severe on stems in solidly planted mature alfalfa fields. Early-growing varieties such as DuPuits are particularly susceptible. In the Pacific Northwest, sclerotia germinate in spring, producing small (0.25 to 0.5 inch in diameter), light brown, stalked discs (apothecia). Spores are produced and can be carried long distances by wind.
Symptoms A cottony, web-like growth develops on stems and crowns in spring. Infected stems wilt, turn brown, become soft and mushy, and totally disintegrate. Rotted crown tissue turns yellow-brown at first, then becomes dark brown and soft. Plants may decompose entirely by late spring, leaving only sclerotia lying on the soil surface. Losses may be attributed to winter injury.
Cultural control
- Keep land free of alfalfa for 3 to 4 years to reduce concentration of the fungus in the soil.
- Rotate to grasses or cereals.
- Cut early when the disease is severe.
- Do not plant too dense a stand.
- Avoid overfertilization especially with nitrogen.
- Deep plow to bury sclerotia.
- Plant seed free of sclerotia.
Chemical control
- Quadris Flowable (Group 11) at 10 fl oz/A. Do not apply more than 15.5 fl oz/A per cutting. Preharvest interval is 14 days for grazing or harvest for forage and hay. 4-hr reentry.
- Regalia (Group P5) at 1 to 2 quarts/A on 7- to 14-day intervals starting prior to infection. Under high-disease pressure, use in combination with another fungicide. Does not benefit from the addition of an adjuvant. 4-hr reentry. O
Biological control
- Bexfond at 7 to 14 fl oz/A. O
Reference Stuteville, D.L., and Erwin, D.C. 1990. Compendium of Alfalfa Diseases, 2nd ed. St. Paul, MN: APS Press.