Answer: It sounds like powdery mildew and while the sprays you're using might protect uninvolved plant parts, it won't make the disease go away. I'd pull out the affected plant which will not only remove the disease pathogens from your garden but will also open your plants up to better air circulation. Powdery mildew is a fungus that favors warm days and cool nights. It typically is more of a problem late in the season, but as you've discovered, it can attack during the summer months, too.
Best wishes with your garden!
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