The unusually wet weather we have been having has caused many problems in cereal forages and alfalfa. Sclerotinia is a fungus which has been causing damage to alfalfa, especially in seedling stands. It thrives in wet conditions. In years past, this disease has occurred mainly in the southern San Joaquin Valley, in areas where there are prolonged dense fogs. Although it has occurred here in previous seasons, it has caused little noticeable damage, and has usually been confined to areas a few feet around in seedling fields with dense growth. This season, damage from this disease has been more widespread throughout the valley, and there have been serious instances of infection here.
The disease causes wilted stems which can be seen from a distance. Closer examination shows a stringy white mat at the base of the plants. This is called mycelium, and these fungal strands infect the stems of the alfalfa, releasing enzymes which degrade the cell walls and cell contents, causing stems to collapse and die. If moist weather lasts long enough, the infection can travel to the crown and the whole plant can be killed. Because seedling plants have a smaller, less developed crown, they are more susceptible to being killed. When warmer, drier weather comes, the fungus forms sclerotia, hard black structures that look like a tiny black pebble about 1/8 - 1/16 inch in diameter. These structures will survive like a seed over the summer and will remain in the soil to germinate when conditions again become favorable.
Although this disease can kill both seedling and adult plants, usually the damage is less than it appears. Even though all the stems on an alfalfa plant may be killed, frequently the crown survives and new, healthy stems emerge. The plant will continue to be productive throughout spring and summer. There have not yet been any studies showing how much production is lost from infected stands after they have recovered.
Anything which will open up the stand and allow air to circulate should decrease occurrence of the disease. On established stands, controlling chickweed and other dense, wet weeds will help. No fungicides are registered to control this disease. Once the weather turns warm and dry, the disease will stop growing. Try to remember that often stands that look very bad can recover and produce acceptable yields the following season
November 5, 1999