Powdery Mildew is a fungal disease affects cucumber, squash, calendula, hollyhocks, zinnias, and many other garden plants. Small patches of grey appear on otherwise healthy looking leaves, and if it is not treated right away, the fungus will spread to cover the whole leaf, and then most of the leaves on the plant.
Infected leaves cannot produce as much food for the plant as they should. Fewer flowers (and fewer fruits) are produced. Plants may become so weakened that they cannot survive winter.
Powdery mildew is produced by a number of different fungi of the order Erysiphales. Their spores are carried by the wind from plant to plant, and tend to settle on wet leaves. If the humidity is high enough, they don’t require the leaves to actually be wet, but this is why Cucurbits like squash and cucumbers should never be watered overhead. Instead, you want to water the soil itself.
Number one, don’t water susceptible plants overhead. Full sunlight and good air circulation will keep plants drier and less prone to mildew. Look for hybrids that have natural resistance to mildew. Compost or bury infected plants so that the spores do not overwinter in the garden.
If caught early enough, when small patches are just forming, try applying Bordo copper sulfate spray to the affected leaves and the rest of the plant. This coats the leaves with a fine, non-toxic powder and may prevent mildew from spreading altogether.
Research is underway to develop an organic milk extract that appears to actually kill the fungus. Hopefully this product will be available soon.