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The white powdery coating on a curcubit's leaves is made up of spreading fungal spores. This affects the plant's performance, with growth, flowering and fruiting all being adversely affected. In extreme cases, the mildew can lead to dieback and even kill a plant. Since there are separate fungal species that affect different plants, an attack on your curcubits won't lead to a garden epidemic. Powdery mildew and powdery mildew on roses are similar problems, with similar solutions.
Solution
Organic
Healthy plants are less susceptible, so make sure they are well spaced and have good air circulation around them. Mulch the area to lock moisture in the ground and give plants a regular drink. The moment mildew is seen, immediately prune out and destroy the affected leaves.
Chemical
Use an all-purpose feed rather than one high in nitrogen, which will generate soft new growth that's more prone to infection. Spray with myclobutanil.
