I had an experience with white powdery mildew on my tomato leaves last winter at our old house, and found an easy solution. I make a mixture of warm water, and about a tablespoon each of baking soda and olive oil in a spray bottle. I spray the mixture lightly on the affected leaves, and it very easily gets rid of the mildew. I recommend spraying the mixture early in the day so it has time to dry out.
You may have noticed that I mentioned that my previous problem occurred on tomatoes in winter, which may have seemed a little strange. My husband and I both love tomatoes, and I cook a lot of Italian food, which means a year-round supply of tomatoes is very much welcome in our house. The weather here where we live in Southern California is generally frost-free, so last year I decided to try to grow tomatoes in winter in our greenhouses. It worked out okay until I closed all of the windows and greenhouse doors to try to keep out a cold storm. The lack of circulation brought on some white powdery mildew. We will be trying out a few different methods this winter to try to keep the greenhouse warm, but still keep some air circulation going.
The white powdery mildew pictured above is actually a recent picture from one of my summer squash plants. All of my vegetables are growing much larger in our new garden than I have ever had experienced in any of my previous gardens, due to the warmer temperature and more sunlight. I underestimated how large my plants would grow this year, and did not space them accordingly.
I have been hand watering everything so far since we have been having some irrigation issues around here (full post coming soon). I always try to set the hose on the soaker setting and water near the base of the plants. One particular day, my back hurt too bad to bend over to move the hose around (I had a bad fall after I tripped on some of my husband’s construction equipment in the kitchen). I figured watering the plants from above for one day couldn’t cause too much harm. The combination of the plants being too crowded and the watering from above brought on some powdery white mildew the very next day. I had to go through and spray all of my squash and tomato plants, and thin out a lot of their leaves. They are all doing much better now that they have been sprayed and thinned out a little. 🙂