On Crops: Cucumbers, melons, pumpkins and squash
Worldwide, especially in cool climates
A disease that only occurs following periods of cool, wet weather, downy mildew begins with light yellow angular spots on leaves. Beneath the spots, a furry gray to purplish mold develops. The ideal temperature range for cucurbit downy mildew is 59-68F (15-20C).
If plants are healthy when this disease hits, they will likely produce a modest crop. Plants that have lost more than one third of their leaves to downy mildew are not likely to produce well.
Choose resistant varieties, which are widely available. Make sure plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying. Rotate cucumber family crops to a fresh site to prevent disease buildup, and use mulch to keep soil from splashing onto plants during heavy rains. Avoid using sprinklers or other overhead irrigation methods if conditions are cool enough for downy mildew.
Clip off and compost affected leaves, and then spray plants with 1 teaspoon baking soda mixed with one quart of water. Cucurbit downy mildew usually disappears if the weather become warm and dry.