Powdery mildew needs plant tissue to overwinter spores. The spores do not remain viable in the soil over winter. As long as you practice good garden sanitation and clean out and remove any infected plant parts that remain at the end of the season, there is no concern about the soil.

It's helpful to understand how powdery mildew works. First, there are many, many different types of PM with very similar appearance but these are by and large host-specific. IOW, the PM that infects squashes and other cucurbits is not the same species that can affect greens, or beans, etc.

PM is mostly due to cultural and climatic conditions, meaning that specific gardening practices can either encourage or deter the development of PM, most specifically air circulation and shade (and of course good garden clean-up). But there is little you can do about climate. Infection is favored by high humidity (50 to 90%) during twilight hours,
and temperatures between 60° and 80° F and dry soil conditions. These conditions create an almost ideal incubation situation and is why this disease typically shows up in mid to late summer when these conditions are most prevalent. PM does not
require moist conditions to germinate and grow and spores can
be killed or inhibited by water on the plant. Sometime just hosing/spraying down the plant well on a routine basis is sufficient to quell the development. Shady conditions (which
are most conducive to retaining humidity) and vigorous plant growth
favor disease development while high temperatures (above 90°F) have an
inhibitive effect.

Thanks for the link and the info.

As it turns out gardengal, you pretty much described the weather here for about 4 days straight. It is on the squash plants only as of now. Thankfully, it is really the only thing I am dealing with. I'm just off the shoreline and the constant breeze and cooler evenings seem to abet it making the garden strong for me.

I have a crookneck that seems to have lost most of its deep greencoloring; it is now pale green/yellowish. I've taken about 15 squash from the plant so it owes me nothing. I'm thinking of just pulling it, although it is still blooming.

I cleaned the beds out as much as possible, surely i left some small roots scattered underground, but it was cleaned up good right arou d the frost setting in.

Chock full of info from both, tyvm!

As was said, powdery mildew doesn't reside/live/remain in the soil. So please stop spraying it.

Most likely the white stuff was a non-damaging fungus degrading the compost that's in the ground, including any roots that were left behind.

No need to replace that soil.

The spores that cause Powdery Mildew travel in on the wind so it is doubtful you can do anything to keep it from returning next year. Control is fairly easy, however, with sprays of 1 teaspoon of baking soda mixed in on gallon of water with a few drops of oil added, or a 50/50 mix of fat free milk and water. Or if you can find Potassium Bicarbonate that is reported to work a bit better. Neem Oil products and Jujoba oil are also reported to be good controls for Powdery Mildew.

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7493.html

kimmq is kimmsr

I have been spraying the plants fairly faithfully. Some of them seem to have stabilized and show no new signs, others like one of the crooknecks, appears to be beyond repair. The leaves are all yellowed, no new growth of leaves or blooms. The zucchini had it on a handful of leaves and seemed fine then all of a sudden, 48 hours, many of the leaves are turning brown after the PM showed up.

Appreciate all the help/input. It showed up in the exact same bed as last year and one that is new as of this spring. I wonder if being down on the shore has some impact on the conditions needed for WPM to thrive.