White rot—Sclerotium cepivorum
White rot affects onions and garlic. Leaves of infected plants show yellowing, leaf dieback, and wilting. Leaf decay begins at the base, with older leaves being the first to collapse. A semiwatery decay of the bulb scales results. Roots also rot. A fluffy white growth develops around the base of the bulb. As the disease progresses, this mycelium becomes more compacted, less conspicuous, with numerous small spherical black bodies forming.
The pathogen persists as small, dormant structures, called sclerotia, in soil. Sclerotia can survive for over 20 years, even in the absence of a host plant. Disease severity depends on sclerotia levels in the soil at planting. As few as one sclerotium per 10 kilograms of soil can result in disease; one sclerotium per kilogram of soil, measurable disease loss; and ten to twenty sclerotia per kilogram, infection of essentially all plants.
Sclerotia can be spread by irrigation water or on plant material, including wind-blown scales. Sclerotia remain dormant in the absence of onion or other related crops.
Disease development is favored by cool, moist soil conditions. The soil temperature range for infection is 50 to 75°F, with optimum at 60 to 65°F. At soil temperatures above 78°F, the disease is markedly inhibited. Soil moisture conditions that are favorable for onion and garlic growth are also ideal for white rot development.
The most effective controls for white rot are avoidance and sanitation. Use of raised beds and careful
furrow irrigation can help limit damage. Space plants well enough to allow for good air circulation. Destroy
diseased plants. To prevent spread in soil, do not compost. Do not replant onions or garlic in that area;
fungus survives in soil for years. Solarization will help control this disease.
Sclerotia and white mycelium on bulbs
and wilting of infected onions