Sclerotium rolfsii (teleomorph Athelia rolfsii)
The fungus infects the outer scales of bulbs resulting in the development of white spot-like lesions. The infected bulb and neck tissues become soft and a watery rot develops. A white fungal growth often develops over the surface of the bulb scales, and mustard seedsized light brown sclerotia form on the infected tissue, as well as in nearby soil and debris.
Conditions for Disease Development
The pathogen has a wide host range and infects as many as 500 plant species besides onion. The fungus can survive for many years as sclerotia in the soil or for shorter periods in infected plant debris. It may spread from plant to plant in the root zone or through the movement of soil and water. Disease is most severe in warm [25-30°C (77- 85°F)], moist soils that are high in organic matter. Fungal growth rapidly decreases below 15°C (59°F), resulting in little disease development.
Deep plowing of crop residue to bury sclerotia, soil fumigation or soil solarization may all help to reduce disease on subsequent crops. Crop rotation to cereals and grasses may help to reduce inoculum levels in soil. Postharvest fungicide treatment of bulbs, as well as, storing bulbs at 10°C (50°F) or lower may help to limit storage losses.