Answer: Based on your description it sounds like they are suffering from powdery mildew, in which case cleaning away the infected parts as you have been is the right thing to do. Usually, the best controls for this are to increase air circulation and reduce moisture sitting on the leaves. Some ways to do that are to plant the plants further apart, thin them individually, and avoid overhead watering. In my experience it also seems to help if the plants are not allowed to dry out, menaing they are kept in a moist yet well drained (not soggy) soil. Some gardeners also resort to use of fungicides. If it happens that the house itself or some other barrier is blocking the available airflow, you might need to consider relocating the garden.
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