Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Sclerotinia crown and stem rot of alfalfa is common in Delaware. Following the wet fall and winter with the amount of snow cover that we experienced, be on the lookout for this disease. It was diagnosed recently in nearby MD. The disease usually causes most damage in fall-seeded stands, but single or groups of plants in stands of any age can be killed. Sclerotinia in alfalfa is favored by cool, wet weather in the late fall and snow cover in the winter. The disease can easily go unnoticed if only scattered plants or small patches in fields are killed, and may be mistaken for winterkill.
Infected alfalfa stems with white fungal growth and black sclerotia of the Sclerotinia fungus.
Sclerotinia crown and stem rot of alfalfa is fairly easy to recognize. If you see dead plants or wilting or dead stems in late March or April, look for white moldy growth (especially in wet conditions) and sclerotia. The telltale sign of infection by Sclerotinia is sclerotia on infected tissue. Sclerotia are small, hard, black fungal structures about 1/8 inch in diameter and nearly round or elongated, up to 1/4 inch or more. If you find dead plants killed by Sclerotinia before they completely decompose, you may be able to find white moldy growth and sclerotia on the dead tissues. In other cases, the infection may have infected the crown but not killed the plants. The crown can be soft and covered partially with white moldy growth, the internal crown tissue will have a yellow-brown color, and sclerotia may be scattered over the surface. Wilting and dead stems are another indication of Sclerotinia infections. The lower halves of stems are most frequently infected, and they also often contain white moldy growth and sclerotia. Some or all stems of a plant may be infected. Material adapted for DE from University of Illinois Extension.
Management of sclerotinia crown and stem rot of alfalfa is based on site selection, planting date, crop rotation, and tolerant varieties. If possible, new fields of alfalfa should be established where there is no history of severe sclerotinia crown and stem rot of alfalfa or red clover. Spring planting allows the plants to develop resistance prior to the time that most infection occurs in the late fall. Alfalfa should not be rotated with red clover. Fungicides are not available for control of this disease. Alfalfa cultivars have been developed that can have increased survival and productivity under conditions of low to moderate sclerotinia disease pressure. These may be beneficial where sclerotinia crown and stem rot is a problem.