Powdery mildew affects all green parts of the vine, is favored by densely shaded canopies and overcast weather, and can reduce yields if infection occurs before flowering.

• A. Powdery mildew affects only the grapes and increases berry size.
• B. Powdery mildew affects all green parts of the vine, preferring densely shaded canopies and overcast weather, and can reduce yields if infection occurs before flowering.
• C. Powdery mildew enhances photosynthesis.
• D. Powdery mildew only affects roots.

Answer: (B)