Powdery mildew starts as small white spots on leaves.
It usually occurs on squash, cucumber, and beans. As it spreads it covers leaves and stems with white powdery coating. Powdery mildew is a true mildew and appears as a powdery or felty white growth on the surface of affected plants. The problem most often occurs during periods of high humidity. This mildew feeds on the plant by means of tiny suckers, draining the plant of important nutrients.