Next year, look for varieties that are resistant to powdery mildew. There aren’t many of them, but after a quick look in a seed catalog, I saw that ‘Cinnamon Girl’, ‘Jill Be Little’, ‘Renegade’, and ‘Gumdrop’ show intermediate resistance to PM.
Also, next year, grow your pumpkins in a different spot. Practicing crop rotation reduces the likelihood that disease spores lying dormant in the soil will infect the next crop.
Keep the foliage as dry as possible. Water in the morning so leaves are dry going into the night. Plant pumpkin vines in an area with good air circulation. Small-fruited varieties can be trained on a trellis, so they’re off the ground.
Avoid planting in cool weather, and avoid any extreme in temperature—hot or cool.
And this is contrary to some advice found on the internet, which claims that dryer soil reduces PM. Actually, dry soil for cucurbits and other plants stresses the plant, lessening their resistance to disease. So, give pumpkins well-enriched soil, deeply dug and well-drained. Keep the soil moist by watering deeply but not daily. And fertilize every few weeks.
Spread a thin layer of pine fines (partially composted bits of pine bark) under and around the plant. Pine fines have been shown to give some protection from pathogens. This material also performs as a barrier between plant and soil and might prevent disease spores from splashing onto the leaves. It also helps keep the soil surface under the plant a bit dryer, while it suppresses weeds and loss of soil moisture.
Check companies that sell garden/greenhouse supplies for organic PM sprays. I’ve used a diluted solution of horticultural oil (not dormant oil) on PM in rosemary, thyme, and other plants with good results.
Good luck. But, if you have pumpkins already growing and maturing, the plants will need only a little more time this season.