Answer: A white powdery substance is a symptom of a fungal disease called powdery mildew. It often attacks plants at the end of the growing season when all the right environmental conditions are present - especially cold nights, warm days and poor air circulation. Without knowing the name of the vine, it's difficult to suggest a control. I'd cut the affected plant parts off to stop the spread of the disease and also to open it up to better air circulation. Be sure your vine is getting adequate sunshine and fertilize it in spring to promote healthy new growth. Best wishes with your garden!
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