To control minor infestations, pick off affected plant parts and either compost them in a hot compost pile or bag them tightly and put them in the trash. Research studies in 1999 and 2003 on infected zucchini and winter wheat (respectively) indicated that spraying cow's milk slowed the spread of the disease. To try this at home, mix 1 part milk with 9 parts water and spray the stems and tops of leaves with the solution. Reapply after rain. Spraying leaves with baking soda (1 teaspoon in 1 quart water) raises the pH, creating an inhospitable environment for powdery mildew.
You can both spray and pour liquid seaweed onto your plant’s leaves. Research has shown that this has a powerful “booster” effect to your plant’s health and it helps fight off the powdery mildew. This is being used in many vineyards now as an organic control because it seems to work particularly well on crops that produce fruit. I mention it as a good alternative.
Sulphur sprays are quite effective at stopping the spread of powdery mildew. Remember that they do knock out beneficial soil fungi as well so do only spray to runoff. You can find sulphur in almost any garden shop.
Sodium bicarbonate (baking soda) is recommended by many gardeners and when it is mixed at the rate of between 2 and 10 g per litre of water (add a small dash of liquid soap as a wetting agent). (1 teaspoon to a quart of water) I’ve seen research that up to 20g / litre of water has worked well with no burning.
And to just to make your day, it has also been reported (I’ve never used this myself) that urine when diluted at 1 part urine to 4 parts water is an effective powdery mildew control. There’s another reason to take a seventh inning stretch.
Milk is another very effective spray for powdery mildew. Mix the milk at a ratio of one part of milk to nine parts of water and spray weekly. Do NOT go higher than 3 milk to 9 water or you’ll attract other fungus problems that want to feed on the milk. Skim milk works well as it contains no fat to turn rancid (and attract other problems that like the smell of rotting fats.)
There are also products on the garden center shelves featuring jojoba oil and neem oil. I can’t speak to these but some gardeners swear by their effectivness for controlling powdery mildew.
Source: Simple Gifts Farm
A while later...
Okay, I made a spay of milk, baking soda, olive oil and water. I've sprayed. We'll see how it goes. I took before pictures so hopefully I'll have dramatic results to share with you soon. =)