Downy Mildew
Downy mildews are fungal diseases that attack many fruits, vegetables, flowers, and grasses. The primary symptom is a white to purple, downy growth, usually on the undersides of leaves and along stems, which turns black with age. Upper leaf surfaces have a pale color. Lima bean pods may be covered completely, while leaves are distorted. The disease overwinters on infected plant parts and remains viable in the soil for several years. It is spread by wind, by rain, and in seeds. To control it, buy disease-free seeds and plants, follow a 3-year rotation, and remove and dispose of infected plants. Preventive sprays of bicarbonate may be effective.