Since a few days, I start to notice some powdery mildew appearing on leaves, probably due to a sudden dip of temperature last week. (It didn’t rain for a long time.) I tried my best to get rid of infected leaves (very early stage), but new ones with spots appear in a speedy manner. I just sprayed this (see below) today on my 40 tomatoes, for prevention, I also treated the melon and the chili. In the past, I have used copper in the past, but since it is quite toxic (even it is still considered as organic), I tried to minimise the use to yearly treatment, like peach tree, olive tree etc.
Baking Soda (sodium bicarbonate) - This is possibly the best known of the home-made, organic solutions for powdery mildew. Although studies indicate that baking soda alone is not all that effective, when combined with horticultural grade or dormant oil and liquid soap, efficacy is very good if applied in the early stages or before an outbreak occurs.
Use this recipe to make your own solution—mix one tablespoon of baking soda with a teaspoon of dormant oil and one teaspoon of insecticidal or liquid soap (not detergent) to a gallon of water. Spray on plants every one to two weeks.
From here, this page lists several treatment: