It's the white powder-like spots on the leaves and stems of squash and cucumbers. We have it at the MUSC Urban Farm, on some of the home victory gardens, and the verge garden.
It's a fungus. A mixture of 1 tbls of Baking soda, 2.5 tbls of vegetable oil, and 4-5 drops of liquid soap added to a gallon of water will act as a natural, inexpensive fungicide. You can also try one part milk to three parts water.
Spray the leaf, underside of the leaf, stem connecting it to the vine, and vine itself.