Keith,
It looks to me like powdery mildew. I have it on my pumpkin and squash vines, too. Never had any luck treating it, but most plants (at least perrenials) seem to do ok anyway. I just cut off the worst leaves. I think the pumpkins will ripen. There are new leaves being formed to produce sugars.
Keski

Yup. That's what it was. Unfortunately, I reacted too slowly. Lost the entire patch...only took a little over a week. Also, it infected my two 4'x4' squares (each one had another different pumpkin plant...one decorative, one pie pumpkins.) I'm still trying to save those...but I'm not optimistic. My canteloupe is affected as well...but I THINK I might be able to save that.

So....this is something that hit QUICKLY, completely out of the blue. What should I plan next year...to make sure not to have this happen again? I am completely heartbroken....nurtured these plants for MONTHS...and just as they were producing....all gone.

So far, my first year of serious planting...

A) Lost half of my potato crop (wilted from high temps, and eaten by horned catepillar);
B) Lost most of tomato plant due to compost tainted with herbicide, and replacements eaten by deer;
C) Lost entire pumpkin crop;
D) Lost cabbage, broccoli, and cauliflower...due to malfunctioning watering system for sprouts (while on vacation);
E) Lost half of strawberry crop (watering issues.)

SIGH!!! This is harder than I thought. Definitely has affected the early enthusiasm, I had when I started.

Keith - I hear your pain. There is a post about powdery mildew on the site a few below ours. It deals with some solutions you can try - like baking soda, etc. Hot, hazy, humid conditions make it perfect for powdery mildew. I've not tried anything for it except the baking soda spray and didn't have any luck with that. Where I live summers are the 3 H's. At least when it is hot. You do have to be vigilant with watering sprouts. Chard and my green beans were bitten off 3 times by deer even right next to Irish Spring soap. Some of my tomatoes got deer damage early. Then I surrounded part of the garden with chicken wire. It seemed to help. The other garden I put bamboo poles tied with fishline at 1'and 3' heights. That also seemed to help. Now we know how hard it can be to produce food. I had squash vine borer which I eradicated with a wire, but production suffered. I only have one butternut and the darn seed cost a couple of bucks plus shipping - definitely in the red on that one. Oh well.
Keski

Yup...it looks like the canteloupe (left section of the patch, in the picture above) is going too. I got four canteloupes in production....but I don't see them lasting long. (sigh) Here's hoping the Fall crop survives - brocoli, cauliflower, cabbage, brussel sprouts (if the sprouts survive), lettuce.

Keith- How do you water your garden? Overhead sprinklers or with a hose, down by the roots?

I'm no expert, since we're in a pretty dry area, but I know roses can get powdery mildew if they are watered overhead or in the evening. I had two Zephirine Drouhin roses that are now at my mom's for that reason.

If you live in a humid area, try watering in the morning and down at the roots. It may make a difference for next year :)

This same disease affected my cucumbers! let me tell you these were BEAUTIFUL plants, they still managed to produce, but not what I expected. It did not affect anything else though! that is what gets me???

Can this powdery mildew be spread by composting affected plants??????

To arborvitea 96 -- YES composting affected plants can spread it! As long as the mildew is still "alive" it spreads via wind. The best thing to do is rip it out and throw it away in plastic bags. I lost my first crop of pickling cucumbers to powdery mildew this year. I was able to slow it down by spraying garlic water on it, but I wasn't able to kill it. A master gardener in our area said it was the combo of low temps/high humidity plus no air circulation underneath plant leaves that made mildew so bad around here this year.

I have read about a product called Serenade being a relatively new, safe, organic solution to powdery mildew and other fungus. Unfortunately, I have not been able to find it in Canada. Has anyone reading here found that Serenade would be worth the money for keeping powdery mildew under control? Powdery mildew wiped out my pumpkins early too this summer.

I have a huge patch and it looks very healthy but I only have one pumpkin. What can I do to get more pumpkins?