ALL cucurbits are prone to powdery mildew....cukes and melons just tend to be somewhat less so......but even they are not fully resistant. And this does tend to be more of a late season phenomenon, as the climate conditions in late summer foster and encourage the development of the disease.

It's a bit late to attempt control via a fungicidal spray as they are more preventative than curative. It is suggested to start spraying as soon as you see signs of any female flowers appearing. For now, I would just concentrate on removing any highly damaged foliage and opening up the vine as much as possible to good air circulation.

^^ Some squash varieties are bred to have some resistance -- butternut at least. I thought some kabocha too, but maybe I'm wrong there. Didn't have a lick of it last year, but I got wiped out by it year before that -- I had to harvest the kabocha early because the vines actually died from the mildew it was go bad, and the flavor hadn't fully developed and they didn't store very well. That is my concern -- I've got a lot of beautiful, tasty-looking squash out there...

I'll have to look into a preventive spray next year if I can't find any resistant varieties of kabocha. Interestingly, the cukes from the seed DH bought had the mildew, but the other vines from the seed I bought didn't have any at all, even though they were intermingled on the same trellis. Maybe the variety I ordered does have some resistance bred into - easy enough to find out by looking at the seed packet (but that would require me having to find it buried somewhere in the big pile of packets I have - maybe some other day when I remember).

I don't know of specific kabochas with mildew resistance. Kobocha is a buttercup type and Bon Bon is listed as having some PM resistance. https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2076&context=farms_reports

Just found four kabochas that claim resistance to PM race 1. Geisha , Golden Butta Bowl, Sake, Speckled Pup PMT. Available from NE Seed.

^^ Thank you!!