Answer: What you describe sounds like powdery mildew and if so, it is extremely difficult to control once established, as you have discovered. Once a plant is fully involved, there's just no stopping the disease. There are some resistant vegetable varieties so be sure to choose those next time you plant. Removing affected plant parts as soon as you see them and using a fungicide can sometimes protect unaffected plant parts, but it doesn't always work. I'd remove the plants from the garden before the disease spreads to nearby plants and choose disease resistant varieties of veggies for next year's garden.
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