Powdery Mildew on summer squash (Erysiphe cichoracearum)
These are no happy days for my summer squash - they are being "harassed" by powdery mildew. There are different species of powdery mildew and they are plant-specific. For example Erysiphe cruciferarum will infect crucifers while Erysiphe pisi prefers peas. The pathogens that cause powdery mildew are unique from most leaf-infecting diseases because their spores do not require free water to germinate. As a result they can infect their host plants in very dry condition such as ours. That is why in a time when blackspots and peach curl are all wiped off by the lack of moisture, powdery mildew is thriving happily.
The strength of this pathogen to withstand the lack of water can be totally reversed by water itself. Hosing the leaves with water will do the trick depending on the severity or stage of the infection.
Some literature on powdery mildew: