White rot of onions

Sclerotium cepivorum is a devastating soil-borne fungal pathogen that specifically infects plants of the Allium genus. As a member of the kingdom Fungi, it causes white rot, a disease that can lead to total crop failure in onions, garlic, and leeks globally, particularly in temperate climates.

The disease is characterized by a fluffy, white mycelial growth on the roots and basal plate of the bulb. As the infection progresses, affected leaves yellow and die back. Small, black, hard structures called sclerotia develop within the fungal mat, serving as the primary survival mechanism of the fungus in the soil for over a decade.

The life cycle begins when host root exudates trigger the germination of sclerotia. The fungus then colonizes the root system, eventually rotting the base of the bulb. Because sclerotia are highly resilient to environmental stress and chemical treatments, the pathogen is extremely difficult to eradicate once a field is contaminated.

Temperature is the critical limiting factor for disease development. The pathogen thrives in cool, moist soils, with the infection rate peaking between 15°C and 20°C. Once soil temperatures exceed 25°C, the fungus enters a dormant phase. However, the presence of these structures poses a constant risk to subsequent susceptible crops.

Effective control requires an integrated strategy to minimize inoculum levels in the soil.

• Implement long-term crop rotations of 8 to 10 years without Allium crops.
• Use certified disease-free sets and seeds to prevent introduction.
• Apply precise chemical seed treatments using registered fungicides.
• Practice good field sanitation by removing and destroying infected plant debris.