What is one of the diseases affecting hops?

Downy mildew and powdery mildew are both significant diseases that affect hops, making the correct answer D, as it encompasses both of these threats.

Downy mildew, caused by the pathogen Pseudoperonospora humuli, is particularly problematic in moist conditions. It affects hop plants by causing yellowing of leaves and leading to a loss of yield and quality, as well as impairing the overall vigor of the plant.

Powdery mildew, on the other hand, is caused by the fungus Podosphaera macularis. It manifests as a white, powdery growth on leaves and can similarly reduce yield and quality by obstructing photosynthesis and affecting hop cone development.

Both of these diseases require careful management in hop cultivation, such as the use of resistant cultivars and fungicide applications, to mitigate their impact on hop production. The presence of either disease could significantly affect a brewer's supply of quality hops. Other options, like tomato blight, are not relevant to hops and do not impact this specific crop.