What is the main effect of downy mildew on grapevines?

• A. Fungi on leaves
• B. Bacteria in soil
• C. Virus
• D. Attacks green parts of the vines, defoliates

Answer: (D)

The main concept here is that downy mildew primarily targets actively growing, green tissue of grapevines, and its most noticeable impact is defoliation. The pathogen is an oomycete that colonizes the undersides of leaves, producing symptoms such as pale-yellow patches and a characteristic white, downy growth. This infection reduces the vine’s photosynthetic capacity and weakens growth, with defoliation being a common and economically important consequence because losing leaves lowers energy production for fruit development. It’s not about bacteria in soil or a virus, and while people often refer to it as a fungus, the organism is classified as an oomycete, which helps explain why the main effect described—attacking green parts of the vines and causing defoliation—best fits.