Tan spots on leaves (first picture) look somewhat like the onset of anthracnose, a fungal disease of cucurbits. Leaves eventually start browning from the edges (second and third pictures). It lives in the soil. Warmth and high humidity encourage it. High heat and dryness discourage it. I think fungicides may help as a preventative. I've never had this problem (blowtorch temps), but I know of others in more temperate climates who have.

I get it every year. Another pic from today

Are you always growing the same variety?

You might be able to find a more disease resistant type...

The local extension office diagnosed mine as downy mildew (not prowdery mildew). I get it every single year. I planted three varieties this summer in three different places and they all got the same symptoms. There are a few varieties that are less susceptible to this disease.

The pix I see online for downy mildew do look very much like what I see here. But again, that's fungal, and fungicides help as a preventative. Cornell says chlorothalonil (e.g. Daconil) and mancozeb (e.g. Immunox) are best. Copper doesn't work well.

I should add that an indicator of downy mildew is that the spots on the undersides of leaves are grey to purplish. Topside they are tan.

Perhaps it's both...?

Thanks guys! It is even on my new plant that just got flowers.