The image on the left is from the APS website. http://www.apsnet.org/publications/im...
So, I assume this is not your plant? Or did you contribute that image to APS.
Is the image on right your image.

It does look like Powdery Mildew. This is a fungal disease. It is caused by many different species of fungi belonging to different genera. It would be impossible to tell form the photo.
Leaf symptoms are very distinctive with white patches of fungal growth (the mycelia) developing on develop on the
leaf surface. The patches get bigger and join together to cover
the entire lower surface.


This disease will reduce your yield and will spread. Unlike many fungal diseases it is quite tolerant of drier conditions and higher temperatures.

How to control it?
1) Remove infected leaves and place them in a plastic bag and dispose of them in the thrash. Do not compost them
2) Using water spray on the leaves washes the spores off. However, this can lead to other disease like water molds so it is not typically advised. In your case the area might be dry so the soil with not remain moist.
3) Use Copper, Sulphur or Potassium bicarbonate foliar sprays. Do you have access to such things
Copper sprays: Spraying plants with copper and sulfur has occurred for over 100 years. It was and still is called Bordeux Mix as it was discovered in Bordeux vineyards (see my blog post from way back https://www.plantvillage.com/blog?loc...)
An example of the product is here http://www.vitax.co.uk/home-garden/vi...
Copper is sometimes considered organic, sometimes not. the RHS have some details here http://www.rhs.org.uk/advice/pdfs/fun...

The reason copper can be problematic is that it is toxic to other organisms and can build up on commercial farms. Recently the EU ruled on it to reduce is use in potato farms http://croplifefoundation.files.wordp......

4) Milk sprays are an organic solution
Milk is often suggested as a control. Somepeople assume that milk must be an old wife's tale but there are studies that show that at high concentrations the applciation of milk to the leaves is better than fungicides in controlling

"Milk applied twice a week at concentrations of 10% and higher controlled powdery mildew on zucchini squash at least as effectively as conventional fungicides"
Wagner Bettiol, 1999 http://www.sciencedirect.com/science/...

this is from the same paper
"Cow milk may have more than one mode of action in controlling zucchini squash powdery mildew. Fresh milk may have a direct effect against S. fuliginea due to its germicidal properties. Milk contains several salts and amino-acids and these substances have been shown to be effective in controlling powdery mildew and other diseases . Several authors have shown that sodium bicarbonate, oxalate, dibasic or tribasic potassium phosphate, and other salts and amino-acids have been efficient in the induction of systemic resistance . Therefore milk may also indirectly affect S. fuliginea by inducing systemic resistance.
Milk is not a potential environmental or food contaminant, consequently it can be used in organic agriculture. Several organic growers have been spraying 5% cow milk once a week to control powdery mildew on zucchini squash and cucumber."

The mode of action is the production of free radicals following the exposure of milk to light. This in turn causes a collapse of the fungal hyphae (the tubes of fungi, which they extend to grow and feed). Here is a paper (behind a paywall, but you can see the abstract) http://link.springer.com/article/10.1...

The application of milk works best on sunny days

Here is some more

https://www.sciencenews.org/blog/food...

5) Besides milk and copper one can achieve success with different salts and clay. All similar in that they effect the growth of the fungus.
For example, baking soda. See this video http://www.youtube.com/watch?v=wg-TH2...
see a nice history here; the idea has been around since 1933 at least https://attra.ncat.org/attra-pub/down...
http://www.clemson.edu/extension/hgic...