Sclerotium rot of onions

Sclerotium rot, caused by the fungal pathogen Sclerotium bulborum, is a significant soil-borne disease affecting various bulbous plants. The pathogen primarily targets the basal plate and roots, causing progressive tissue decay that can result in total crop failure if left unmanaged. Its ability to persist in the soil for several years through sclerotia makes it a difficult challenge for growers.

The disease primarily affects Allium species, including onions, garlic, and shallots, as well as several ornamental bulbs. The fungus infiltrates the host through the root zone, often exploiting natural wounds or stress points in the plant’s anatomy. The damage is often most severe during cool, moist periods that follow the initial planting stage or during the ripening phase.

Symptoms initially manifest as chlorosis and wilting of the leaf tips, which gradually progress to the entire foliage. Upon inspection of the bulb base, one can observe white, cottony mycelial growth accompanied by the development of small, spherical, black sclerotia. The root system eventually decays completely, causing the plant to detach easily from the soil.

Optimal conditions for disease development include high soil moisture and moderate temperatures, typically between 15°C and 22°C. The pathogen spreads effectively through contaminated soil particles, agricultural tools, and infected planting sets. Because sclerotia can remain dormant for extended periods, the fungus can survive multiple seasons without a susceptible host present.

Effective control strategies focus on integrated pest management (IPM). This includes strict crop rotation, ensuring a minimum interval of 4–5 years between susceptible crops, and sourcing pathogen-free bulbs for planting. Soil solarization and the application of fungicides, where permitted, can significantly reduce the inoculum density in the field, protecting future harvests.

• Ensure proper soil drainage to prevent fungal proliferation.
• Remove and destroy infected plant debris promptly.
• Use heat or chemical treatment for infected planting materials.
• Implement strict sanitation measures for field equipment.