Onions left in the refrigerator for too long will often exhibit a fuzzy white growth around the roots. This may be onion white rot and these infected plants should never be allowed to come into contact with anything related to your garden.
Onion white rot lifecycle
This fungal disease is caused by Sclerotium cepivorum and it strikes onions, garlic, and other members of the Allium family. Unlike other fungi, this one does not produce spores. Onion white rot can stay viable in soil for over 20 years! Instead, it survives in the soil as sclerotia (dormant fungal structures with stored food). These sclerotia can travel by piggybacking on equipment, people, tools, infected plants and soil, and even on the wind. A single sclerotium in 20 pounds of soil will infect nearby plants. One or two sclerotia in one pound of soil will cause all plants to be infected. The sclerotia will stay dormant in the soil until onions, garlic or other Allium are planted. As part of their normal growth, these plants discharge certain chemicals into the soil around their roots. These chemicals stimulate the fungi to germinate. Soil temperatures of 50° to 75°F allow the fungi to grow, peaking between 60° to 65°F.
Onion white rot symptoms
Onion white rot first appears as a cottony growth around the base and sides of onion or garlic. This is the fungal mycelium (vegetative growth). This mycelium then becomes more compacted and sclerotia become visible as poppy seed sized black spots. Other signs of infection are yellowing lower leaves, wilting, and leaf dieback. Older leaves will begin to rot at the base and roots will decay, making it easy to pull infected plants from the soil. Host plants die soon after infection. Onion white rot tends to travel sideways, from plant to plant, using intertwined roots as a pathway. Onion white rot thrives in cool, moist soil.
How to control onion white rot
This fungal disease is extremely persistent in the soil and there are no known chemical controls. Once an infection has occurred, all tools and equipment that have come into contact with the soil should be sanitized with a 1 part bleach, 9 parts water solution. Some of the crop may be saved by using the least amount of water to keep the plants alive.
Infested soil should not be planted with onions or garlic for at least 8 - 10 years. If that is not an option, starting onions from seeds, rather than sets (transplants), may help. Seeds will have minimal root growth while the fungus is most active. Onions grown from seed take two seasons to mature, so the problem may not be avoided entirely. Also, spacing onions 12 or more inches apart may halt the spread of this fungus. Leeks are the least susceptible members of the Allium family.
Due to the direct link between onion and garlic chemical fumes (exudates) and fungal germination, research is being conducted to see if applying Allium plant hormones to infected soil without a host crop might trick the sclerotia into germinating at a time when there is no food for them. Since this is not currently an option for the home gardener, prevention is the best course of action.
To avoid infesting your soil with white onion rot, be sure to plant only certified pathogen-free onion sets and garlic cloves. Since onions and garlic purchased from grocery stores are not guaranteed pathogen-free, it is best to keep them out of the garden and the compost pile.
You can grow a surprising amount of food in your own yard. Ask me how!