Onions left in the refrigerator for too long often exhibit a fuzzy white growth around the roots. Those onions have a fungal disease, onion white rot. Keep them from coming into contact with anything related to your garden.
Onion white rot lifecycle
Sclerotium cepivorum fungi strike onions, garlic, and other Alliums. Onion white rot can stay viable in the soil for over 20 years. Unlike other fungi, this one does not produce spores. Instead, it survives in the soil as sclerotia (dormant fungal structures with stored food). These sclerotia can travel by piggybacking on equipment, people, tools, plants, and wind.
A single sclerotium in 20 pounds of soil will infect nearby plants. One or two sclerotia in one pound of planting soil will cause all plants to be infected. The sclerotia will stay dormant in the soil until Allium comes on the scene. As part of their 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. These fungal mycelia are vegetative growths that become compacted. These poppy seed-sized black spots are visible to the naked eye. 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 travels sideways, from plant to plant, using intertwined roots as a pathway. This problem thrives in cool, damp soil.
Onion white rot control
This disease is persistent in the soil. And there are no known chemical controls. Once an infection has occurred, sanitize all tools and equipment that may have come into contact with it and avoid planting Alliums in that location for at least 8 - 10 years. If that is not an option, starting onions from seeds rather than sets (transplants) may help. The 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. You may be able to save some of your onions with deficit irrigation. Leeks are the least susceptible members of the Allium family.
Due to the direct link between onion and garlic chemical exudates and fungal germination, research is underway to see if applying Allium plant hormones to infected soil without a host crop might trick the sclerotia into germinating when there is no food for them. Since this is not an option for the home gardener, prevention is the best bet.
Plant only certified pathogen-free onion sets and garlic cloves to avoid infecting your soil with white onion rot. Since onions and garlic purchased from grocery stores are not guaranteed pathogen-free, keep them out of the garden and the compost pile.
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