Powdery mildew

Figure 20.
Powdery mildew on cucurbit leaves.

The dry, humid and dense plant growth within a high tunnel is optimal for development of powdery mildew. Powdery mildew (Podosphaera xanthii) is a fungus that looks like a white mold on leaves (Figure 20). The disease appears on the lower leaves of the vine and gradually spreads through the canopy. The vines become weakened from leaf loss, and fruit size can be significantly reduced. Plants should be regularly inspected, starting at fruit set for powdery mildew. Choose resistant cultivars (Table 3). The high tunnel should be properly vented to reduce relative humidity. Many effective fungicides are labeled for powdery mildew control.