Its definitely pm. It is easily cured with a diluted milk/water mix. 1/10 ratio. I have had this problem, and the milk mix took care of it.
The plants also benefit from the calcium in the milk. You are basically foliar feeding the plant calcium while you kill the pm.
Milk has long been popular with home gardeners and small-scale organic growers as a treatment for powdery mildew. Milk is diluted with water (typically 1:10) and sprayed on susceptible plants at the first sign of infection, or as a preventative measure, with repeated weekly application often controlling or eliminating the disease. Studies have shown milk's effectiveness as comparable to some conventional fungicides,[SUP]
[11][/SUP] and better than
benomyl and
fenarimol at higher concentrations.[SUP]
[12][/SUP] Milk has proven effective in treating powdery mildew of
summer squash,[SUP]
[12][/SUP]
pumpkins,[SUP]
[11][/SUP]
grapes,[SUP]
[13][/SUP] and
roses.[SUP]
[13][/SUP] The exact mechanism of action is unknown, but one known effect is that
ferroglobulin, a protein in
whey, produces oxygen
radicals when exposed to sunlight, and contact with these radicals is damaging to the fungus.[SUP]
[13][/SUP]