Erysiphe and Sphaerotheca species
This common fungal disease most often attacks the cucurbit family and beans. It is most often found in mid to late summer and unlike most fungi it doesn’t need a lot of moisture. It starts out as small, white, round, powdery spots on the top sides of older leaves, but quickly covers whole leaves (top and bottom) and stems. In severe cases the leaves turn brown and die. It also occasionally infects fruit, but more often the damage to fruit is caused by sunburn due to loss of leaves (this also reduces fruit production of course). Powdery Mildew doesn’t usually kill the plant outright, though it slows it down and makes it less productive.
Powdery Mildew is most problematic in shady, humid, areas with poor air circulation, so give your plants good air circulation (provide support for climbing varieties is important). Remove infected leaves to reduce the spread. The fungal spores overwinter on plant debris, so clean up the beds in fall. Also make sure the plants are well fed and watered. Some cucurbit varieties are resistant (marked PM). You can try controlling the infection by spraying affected leaves with compost tea or urine (diluted with 4 parts water).
Image: David B. Langston, University of Georgia, Bugwood.org