A: It's powdery mildew, a fungus that thrives when humidity is high. Minimize the risk of infection by watering early in the day so that soil and leaves dry by nightfall. If it appears early in the season (unusual), control the spread by clipping off infected leaves. Late in the season, it doesn't reduce harvest, so it's mostly a cosmetic issue. Don't compost infected plants at home; send them to a municipal operation, where the compost piles get hot enough to kill spores.
Shown: Sugar snap peas are easy to grow and popular with kids. Pick a variety that suits your spot. The original grows as a vine and requires a trellis or cage for support; bush varieties may not.