White Mold in Alfalfa

April 30, 1999
Matt Montgomery of the Sangamon/Menard Extension Unit reports white mold in third-year alfalfa, and Robert Bellum, Crop Systems Educator, Edwardsville Extension Center, also reports white mold in Macoupin County in first-year alfalfa. The disease in a first-year stand represents the more serious situation.

White mold of alfalfa is also known as Sclerotinia crown and stem rot. Either Sclerotinia trifoliorum or S. sclerotiorum causes the disease. Symptoms appear in the field as small to large patches of dead and dying plants, most frequently early in the season. Initial field symptoms are yellow leaves and stems. This progresses to a wilt and ends as a mushy rot of the lower stem and crown. After the plant has died, the white fungus grows over the plant as well as the surrounding soil. This is hard to miss. Survival structures of the fungus are called sclerotia; they are hard, black, irregularly shaped bodies that look like soil peds about 8 to 10 mm in diameter. The sclerotia can remain associated with the dead tissue or fall to the soil surface. In either case they produce tiny mushrooms (apothecia) in cool, moist weather. Spores are produced by the apothecia and infect nearby plants. All ages of plants are susceptible to the disease. Infection by S. trifoliorum takes place in the fall. Seedlings are more vulnerable to infection, so fall-seeded alfalfa stands are in more danger from this species. Snow cover also favors development of the disease. Infection from S. sclerotiorum typically occurs in the late spring or summer.

Although there is no resistance to Sclerotinia crown and stem rot, there are differences in susceptibility of alfalfa cultivars. Obviously, well-adapted varieties are your best bet for avoiding disease in a situation like this. No fungicides are registered for control of the disease. Since the disease survives as sclerotia on the soil surface, minimum tillage presents an increased chance of disease because only sclerotia that are on or just below the surface produce apothecia.--Suzanne Bissonnette

Author: Suzanne Bissonnette