The first pic was Muskmelon. This is one of my watermelons.

more watermelon damage

And this is a leaf from the pigweed that is growing next to the melons. It is yellow and sick, too.

Are you sure it's not just drought and lack of nitrogen? The weeds and melons won't get the same disease. I'd say apply more water and nitrogen.

Last year in our awful drought my melons got a bad case of powdery mildew, entirely different disease.

Your lack of fruit set could also be related to lack of water. I water mine with about 2 inches every 5-7 days. When they start wilting midday, they need a deep watering. A little every day probably isn't as good.

I think you said they wilt every day. That's probably too dry. After a good deep watering they shouldn't wilt mid day even at 105F.

Maybe it is powdery mildew.

I water them every day and sometimes let the water run for hours. It gets at least one hour a day minimum. The ground is soft under the plastic and when I pull a weed from it, the soil seems to be nice and moist. The muskmelons wilt from the heat even while the water is on.

The pigweed might have a nitrogen deficiency, but I would think that had something to do with drying out from the heat being the real cause of it, rather than a deficiency in the soil.

From what I am reading, one of the types of powdery mildew actually prefers hot and dry conditions and hates to get wet. This year would be the perfect conditions, since it never rains.

http://westernfarmpress.com/powdery-mildew-melons

"P. xanthii prefers dry conditions, with relative humidity of 50 percent to 90 percent for spore production. Although relative humidity doesn�t often reach those levels consistently in the desert, dense foliage and irrigation may create microclimates that favor powdery mildew development. However, presence of water on the plant surface may wash off and/or inhibit the germination of the fungal spores. P. xanthii grows on the outside of leaves, first appearing as a small white growth, eventually becoming a layer of white fluffy mycelium and spores on the upper or lower side of leaves. P. xanthii does not grow systemically (able to spread through the inside of the entire plant), instead this fungus prefers to produce mycelium on the outer surface, and penetrate the leaves with specialized feeding cells called haustoria. The fungus will then produce spores in the absence of free water; spore production gives the leaves the characteristic powdery appearance. The spores are spread to new infection sites by wind. After the fungus has extracted nutrients, the plant cells die, causing the leaves to become brown and papery. Fruit produced by the plant may accumulate less sugar, have reduced yield, and shortened production times."

Robbie:

Not many things like water every day. That can cause the roots to rot which causes loss of leaves. Don't water your melons more than once every 3 days. If you can't extend the interval that long at least you are wetting too small an area around each plant.

I'd say powdery mildew. I'm seeing it on a few things. Around here the lilacs get it really bad.

And now all of my pumpkins have it as well. It does look powdery in this pic.

Some of the young pumpkin plants are already completely dead. I sprayed daconil on them tonight. I walked over to our other gardens, and those plants have it, too. I saw it on our zucchini squash. I don't ever remember having problems like this. I'm only 36, but my grandparents are 80 and feel the same way. It has to be the weather.