last year my stark blue ribbion plum tree had powdery mildew how can i prevent this from happening?
I use a homemade Bordeaux mixture to treat powdery mildew all over my yard and fruit trees. It contains hydrated lime and copper sulfate. you can pick up enough of the supplies to make 100s of gallons of the stuff for about $20 dollars at any feed store. I spray my entire growing area every year during dormancy. I also spot treat during the growing season. The link I posted will give you more details. good luck
Here is a link that might be useful: Bordeaux