It's best to spray before disease onset.. ie, as a preventive maintenance, starting when they're transplanted outdoor.. the most effective is potassium bicarbonate, it can kill it even after onset. no idea why it's not working for you. i spray it weekly and after it rains.. PM has nothing to do with watering your plants. high humidity and warm temps is all you need, although the wetting of leaves can accelerate it. best to trim at least 12-18 inches of foliage from the bottom of the plant to minimize soil to leaf contact (splashing, etc)..

i'd give sulfur or metal fungicide a try then (copper).

There are various factors that need to be present for PM to occur, most related to weather. Moderate temps and high humidity are key factors but dry and warm soils also have a significant role. Making sure the soil stays consistently moist (not allowed to dry out) and mulching can help with that. Maintaining good air circulation and adequate sunlight will also retard development.

There are many different species of PM but most are adverse to water. That means water on plant foliage has no effect on the spread of the disease and will actually prevent spore germination. One of the simplest deterrents is to spray down susceptible plants early in the day to wash off any potential infection. But early enough so that everything can dry sufficiently and any associated humidity abate.

There are lots of fungicidal applications that will work but most work best if applied before any symptoms are present. Most are only prophylactic and not curative and need to be repeated frequently when the conditions are right for the disease. And because PM can quickly build up a resistance to a specific fungicide, it is best to rotate treatments with at least two different types.

And of course whenever possible, select PM resistant varieties.

Good idea starting before it comes on. I will give that a try.

My strategy is to choose resistant varieties. Especially squash -- otherwise it seems to be a losing battle.

I have tried that but it does not seem to help much.