Two things I use to keep Powdery Mildew off the plants that are susceptible are 1) mix 1 teaspoon baking soda in 1 quart of water, or 2) make a 50/50 mix of fat free milk and watrer. These are both sprayed every 5 days to control PM on plants already covered or every 7 to 10 days to prevent PM from getting started.
Dig in the soil and see what you have because a good, healthy soil will aid greatly in growing strong, healthy plants that can better ward off plant diseases and pests.

I'm getting some decent results fighting PM with apple cider vinegar spray. Three TB vinegar to five liters of water. A quick spray on susceptible plants every three to five days, depending on weather, seems to be working.

First I've heard of using an acidic substance to control Powdery Mildew which seems to grow better on acidic leaf surfaces. The baking soda and milk work at controlling PM because they change the pH of the leaf surface to alkaline which makes an environment the PM cannot live on. Be interesting to know if others have similar results with vinegars, which usually kill plant leaves.

My gosh, what great information. I'm a new rose grower. Just sticking whatever I can find into the ground and watching it grow. I purchased two Showbiz the other day from Lowes (coldn't pass up $7.50/per). I am not familiar with powdery mildew but I think they have it (know all too well about blackspot though). I'll give the baking soda and or milk mixed with water a try and see if it has an effect on these wonderful roses. Thanks!

Hi Kimmsr. I started with the apple cider vinegar solution after seeing it here on gardenweb, mainly because I happened to have a quart bottle of it! My PM problem is not extensive--only four to five shrubs affected and only one of these with PM on every leaf. My Mm Isaac Pereire gets curled leaves and puny blooms. The vinegar solution definately arrests the progress--the day after a spray it is pretty much gone. My guess is that the specific ph that PM needs can be altered either way--with an acid or a base. Now I'm trying the Cornell Formula on some roses and the vinegar solution on others (trying to find roses equally affected). I'm getting obsessed! A question: Does it completely nullify your efforts when you spray in the evening and it rains the next day?

I'm experimenting next with the milk and water. Will compare the three: the Cornell, the vinegar water and the milk water. Does the milk water make your garden smell at all bad?

Thank you everyone for responding to my question! KIMMSR, I was under the impression that the theory behind the milk (I thought I read this somewhere?) was that it contained "healthy" bacteria or other organism that would inhibit the powdery mildew. Toward that end, I thought that raw milk must work versus pasteurized, but perhaps it's only ph? No one seems to ever mention raw milk, so perhaps ph is the only mechanism. Any thoughts? Again, thank you everyone for the help. Amy

The original research on milk did use raw, whole, milk, but very few of us, in the USA, have access to raw milk unless we have our own source, hence the use of the fat free milk because some researh indicates that the pasturaization process changes the fat globules in the milk and they apparently do harm to plants. Pasturized milk will not have many, if any, bacteria healthy or not since the temperature is meant to kill those wee critters adn the good bacteria or easier to kill off than the disease pathogens would be.

Here's a crazy idea. What about using yoghurt instead of milk? It's full of good bacteria. Of course, the sour taste must mean it's acidic--but maybe that would still alter the ph, though in the other direction, and disturb the necessary enviornment for fungus... What do you think? (I have definately had some good results with vinegar water and PM)

I've not found anyone, yet, that has tried Yogurt for anything other than eating, although I have seen Pasturized Yogurt out there now.