This disease attacks living plants but is often not noticed until onions are in storage. Sunken, dried areas appear at the neck of the bulb and gradually move down through the entire bulb. Symptoms usually develop after bulbs have been in storage for a month or more. The fungus penetrates leaf tissue, but only affects the bulb. If necks are succulent at harvest time, they provide an entry point for the fungus. The fungus cannot attack a well-dried neck.
Do not apply a nitrogen fertilizer after 8-9 weeks from seeding. Space plants to allow for good air circulation. Spray a biofungicide containing the disease-fighting bacterium Bacillus subtilis every 7 days after plants emerge. Also, a regular spraying schedule with a copper soap fungicide will reduce infection. (Due to the continual emergence of new leaves from the neck, timely and thorough spraying is necessary to keep new growth covered.) We recommend Plant Guardian™ Biofungicide Wettable Powder and Soap-Shield Flowable Liquid Copper Fungicide.