Hello Linda,

Powdery mildew generally occurs from mid-July until the end of the season. Development on tolerant varieties will vary from year to year. Planting tolerant varieties will help delay the development of powdery mildew and improve the performance of fungicides.

Make first application when powdery mildew is observed in the area or is detected by scouting. Add powdery mildew specific products to basic preventative program. Treat until harvest is done.

Dr. Meg McGrath at Cornell’s Long Island Horticultural Research and Extension Center studies show that sulfur and ‘stylet’ oil are the most effective materials for managing powdery mildew. Fixed copper fungicides have overall been less effective, but can be highly effective, perhaps reflecting variation among formulations.

There is some research that indicates a 5% to 10% milk solution sprayed on plants twice a week is effective. See https://plantvillage.psu.edu/posts/4702-pumpkin-white-pumpkin-leaves

I am providing you with a couple Extension articles on managing powdery mildew for organic growers, which should meet your needs. Apply any product according to the label, and avoid flowers as much as possible.

https://www.uvm.edu/vtvegandberry/factsheets/powderymildeworganic.html

http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cuc_Cntct_Fcides.htm

These articles list varieties resistant to powdery mildew, so you can consider them for next season—

http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Pump_PM_Resistant.html

https://blog-fruit-vegetable-ipm.extension.umn.edu/2021/02/2021-considerations-preparing-for.html