Spots on the leaves become covered with a lavender or purplish mold. Requires cool temperatures and rain or high humidity. Under these conditions, onion leaves bend, resulting in lower yields and bulb quality.
Grow onions in well-drained soil to keep roots from getting soggy. To prevent the spread of disease, wash tools and shoes after you work in the vegetable patch. Remove weeds and garden debris, which harbor insects that transmit diseases. Check plants every few days for signs of damage and destroy infected plants. Spray regularly with a copper soap fungicide to reduce infection. (Because of the continual emergence of new leaves from the neck, timely and thorough spraying will be required to keep new growth covered.) We recommend Soap-Shield Flowable Liquid Copper Fungicide.