Which statements describe the optimal conditions for downy mildew and its typical symptoms?

• A. Heavy rainfall and temperatures around 20°C, with yellow circular oil spots on the leaf surface followed by white downy growth on the underside
• B. Dry weather and temperatures above 30°C, with brown patches on the upper leaf surface
• C. Mild humidity with cool temperatures, causing black spots on berries
• D. No rainfall and high winds, with leaf curl

Answer: (A)

Downy mildew on grape leaves thrives when leaves stay wet from rain or heavy dew and the temperature is in a cool to moderate range, around 18–20°C. The pathogen requires leaf moisture to germinate and infect, so wet conditions greatly favor infection and the spread of the disease. Early on, you see yellow, oil-like spots on the upper surface where the tissue is affected. As the disease progresses, a white, fluffy downy growth appears on the underside of the leaf, representing the sporangia-producing mold of the pathogen. If wet conditions continue, these lesions can expand and leaves may become necrotic or drop.

This is why the statement describing heavy rainfall and around 20°C, with yellow circular oil spots on the upper surface followed by white downy growth on the underside is the best fit. The other scenarios describe conditions and symptoms not consistent with downy mildew, which relies on leaf wetness and moderate temperatures to develop its characteristic underside growth.