Onion white rot is a fungal disease that affects plants within the Allium family, including onions, garlic, and leeks. This soil-borne disease can cause significant decay in affected plants, rendering them unusable and reducing crop yield.
Identifying Onion White Rot
Onion white rot often first appears as yellowing and wilting of leaves, particularly the older foliage. In dry conditions, wilting is more noticeable, but in wetter soil, plants might simply become loose and topple over due to decayed roots. Upon inspection, a characteristic white, fluffy fungal growth becomes visible on the base of the bulb and surrounding roots.
As the disease progresses, small, black, spherical structures, known as sclerotia, develop within this white fungal mat. These sclerotia are roughly the size of poppy seeds and are a distinctive sign of white rot. Unlike other onion diseases, white rot specifically targets Allium species, causing roots and bulbs to rot, often resulting in watery decay.
How Onion White Rot Spreads and Survives
Onion white rot is caused by the soil-borne fungus Sclerotium cepivorum, also known as Stromatinia cepivora. This fungus persists in the soil primarily through hardy resting structures called sclerotia, which can remain dormant for many years (up to 20-30 years). Sclerotia will germinate when stimulated by specific chemical compounds, such as volatile sulfur compounds, released by the roots of Allium plants.
The fungus thrives in cool, moist soil conditions, with optimal temperatures for sclerotia germination ranging from approximately 9°C to 20°C. Once germinated, the fungal mycelium grows through the soil, infecting plant roots and progressing into the bulb. The disease spreads through root-to-root contact, or by movement of contaminated soil, infected bulbs, gardening tools, and water runoff.
Managing Onion White Rot
Managing onion white rot requires a multi-pronged approach, as the sclerotia’s long survival in the soil makes eradication challenging. Strict sanitation practices are important, including thoroughly cleaning gardening tools and footwear to prevent spread of contaminated soil. Removing and properly disposing of all infected plant debris (e.g., by burning or landfill) can help reduce the pathogen in the soil.
Long-term crop rotation is a beneficial practice, involving avoiding planting Allium crops in infested areas for 5 to 20 years. While rotation alone will not eliminate the disease due to the sclerotia’s persistence, it helps prevent an increase in the pathogen’s population.
Using certified disease-free planting material, such as onion sets or garlic cloves, is also a preventative measure to avoid introducing the fungus into new areas. Improving soil drainage can create less favorable conditions for the fungus.
Unfortunately, there are currently no widely available onion or garlic varieties with significant resistance to white rot for home gardeners. Chemical treatments are sometimes used in agricultural settings, but their effectiveness can be limited for home gardeners due to the fungus’s resilience and the difficulty of reaching the pathogen in the soil. Early detection and prompt removal of infected plants can help prevent further spread within a garden bed.