How to Manage Pests
UC Pest Management Guidelines
When active, the fungus that causes Sclerotinia stem and crown rot is easily identified by white, cottony, mycelial growth on crowns or stems. In seedling fields infected plants wilt and die, often causing stand loss. For established fields, infected stems wilt and die. The disease can grow into crowns, but plants usually recover.
The disease starts as small, white patches on stems and then spreads to other parts of the plant. Diagnosis can be confirmed by the presence of black, hard resistant structures (called sclerotia) that look like peppercorns. They can be round or irregular in shape and, when broken open, have a white interior. Sclerotia are found at the base of stems, on soil near the crown, or inside infected stems. Dead stems are hollow and easily flattened between the thumb and fingers, making it easy to feel sclerotia, if present, inside. Sclerotia found in stems are elongated in shape.
Sclerotinia stem and crown rot is a cool-season disease. In wet or foggy winters, this disease can be serious on stands planted in September and October, especially when rapidly growing plants form a dense canopy in which high humidity is favorable for disease. Weeds, such as chickweed, further encourage disease by prolonging moist conditions in the canopy.
All the stems of a plant may be infected and die, which makes the plant appear to be dead. But crowns may still be alive and healthy regrowth may appear later in spring, especially in established plants. If plants are young, weakened by stress or other factors, or if favorable conditions for disease exist long enough, entire plants may be killed.
The best strategy for established fields is to remove as much foliage before winter as possible by mowing or grazing. Deep plowing of fields will prevent germination of most sclerotia; however, neighboring fields of alfalfa or weeds hosts (pineappleweed, sowthistle, groundsel, mayweed, mustards, radish, legumes, etc.) can be the source of new infections. Good weed control reduces hosts and opens the canopy, allowing air movement and sunshine at the base of plants, thereby reducing humidity and moisture required by the fungus to start and maintain infections. In dry winters, this disease is not a problem. There is no effective genetic-based resistance incorporated into commercial varieties at this time.
Early February plantings usually escape disease, but growers who prefer to plant in September and October face potential damage from this disease in the first winter. Research has not shown conclusively that using herbicides to burn back seedling growth is effective in reducing disease. The disease may thin stands and, on rare occasions, replanting may be necessary.
UC IPM Pest Management Guidelines: Alfalfa