How to manage this common plant disease
Crops affected: cucumbers, pumpkins, squashes, melons, and in some cases, tomatoes, carrots, grapes)
Appearance: The top sides of leaves look like they have been dusted with flour. As this fungal disease advances, leaves become grayish-green and become dry to the touch. Eventually these leaves turn brown and fall off.
Mechanism of plant damage: Powdery mildew cripples plants’ ability to conduct photosynthesis, and the leaf’s gas exchange system. New growth stops, old leaves fall off, and the plants struggle to stay alive.
Warm temperatures and heavy rains can trigger outbreaks of powdery mildew.
Prevention:
- Choose powdery mildew resistant varieties.
- Plant at proper spacing, and prune plants for good airflow.
- Do not water from above; water the roots instead.
- Starting in early summer, spray plants every 10 days with a mixture of one part milk to four parts water. Any kind of milk could be used, but some report that using non-fat milk can prevent unpleasant smell.
Management:
- Remove affected leaves, plants (if no longer producing or heavily infected); throw in trash to prevent spore spread.
- Organic fungicides could be used (more details in comments).
- Numerous studies suggest that spraying with a solution of 40% milk and 60% water is as effective as fungicides. I will post a link in comments for more details.