Summer is perfect for cucumbers, but it's also perfect for powdery mildew in our climate. We have scorching day temps and very cool humid nights. The mildew loves that sort of combination. This leaf was attacked by the mildew, then I sprayed a Potassium Bicarbonate, Garlic juice, orange oil and biodegradable dish soap concoction on and it seemed to stop the mildew from spreading.
As a request from Kory, I am posting my anti-powdery mildew spray recipe:
1 gallon water
4 tsp Potassium Bicarbonate powder
1 TBS of biodegradable dish liquid
1 TBS of orange oil
4~5 cloves of garlic minced very fine or pureed.
Let the garlic mush it steep in a small amount of water for about 30 minutes then strain out the pulp, then mix the liquid into the rest of the mixture.
Mix all ingredients thoroughly then spray with a regular pump sprayer you can find at Home Depot type stores.
You can buy Potassium Bicarbonate powder online from "do it yourself" wine making shops. I found mind on e-bay.
Don't use any dish liquid that has Sodium Laurel Sulfates (as a matter of fact anything with the word Laurel or Sulfate in it, even if says it's biodegradable, it's toxic to plants.
Spray on the plants when it is cool, not hot, as it will scorch the plants. Also, try not to spray on new shoots as they can scorch too. Prevention seems to be better. My Japanese Cucumber was barely infected when I sprayed, and they are still doing very well. The first sign of Powdery Mildew, spray and it should keep for a good while.