Powdery Mildew. Very common. Tends to hit and kill the older leaves first but it will spread. I just let it run it's course so I can't reccommend any sprays but they exist. For me it usually hits after fruits have set, which are still capable of reaching maturity.

Yes, I just read that after I made my post. The first article said there was no cure so I was doomed. But then I came across another that said fungacides will still work, and that I could save my plants with milk & baking soda, etc.

I will try to do some repair this Sunday and we'll see how these gimmicks go.

There are many copper compounds that are used as fungicides. The most common is Bordeaux mix which is a combination of copper sulphate and hydrated lime. It is accepted in organic farming provided that the number of applications is strictly followed and a proper soil amendment is observed to prevent copper accumulation in the soil.
Bordeaux mix sprayMaterials needed to make a gallon mixture
3 1/2 tbsp of copper sulphate
10 tbsp of hydrated lime
1 gallon of water (4 liters of water)
Wooden stick
Plastic bucket
How to prepare?
1.Add copper sulphate and hydrated lime in water. Make sure to use plastic container.
2.Stir well using a wooden sick or ladle.
3.Protect self from direct contact with the solution.
How to use?
1.Spray plants thoroughly preferably early in the morning, in a dry and sunny day. In this way, the plants have the time to dry and the solution can not penetrate into the leaves' tissues
2.Constantly shake the sprayer while in the process of application to prevent the solution from clogging
Pest controlled
1.Flea beetles on tomatoes and potatoes
2.Anthracnose
3.Bacterial blight
4.Bacterial wilt
5.Black spot
6.Downy mildew
7.Late blight on solanaceous crops
8.Powdery mildew
9.Rust
10.and many other disease causing pathogens
Standard procedures for the preparation and application of homemade extracts
1.Read and follow the label instructions carefully. Ask for assistance from your local agriculturist office when using copper for the first time.
2.Monitor plants regularly and spray only when necessary as copper can accumulate into the soil.
3.Spray in the early morning or late afternoon.
4.Use utensils for the extract preparation that are not use for your food preparation and for drinking and cooking water containers. Clean properly all the utensils every time after using them.
5.Do not have a direct contact with the crude extract while in the process of the preparation and during the application.
6.Make sure that you place the extract out of reach of children and house pets while leaving it overnight.
7.Harvest all the mature and ripe fruits before extract application.
8.Always test the extract formulation on a few infected plants first before going into large scale spraying. When adding soap as an emulsifier, use a potash-based one.
9.Wear protective clothing while applying the extract.
10.Wash your hands after handling the extract.

I picked up some baking soda today and put together the Cornell solution. Sprayed close to a litre worth on my leaves and I'll probably try again on sunday.

Seems this damn disease has officially spread to EVERY ONE of my pumpkin plants, which makes me feel better now that I planted more than I felt I had room for.

I've read so much mis-information on this stuff out there, even on credible sites. Everything from this wont work, to it works only to prevent, to it works and repairs/cures. I also noticed some writers can't even tell the difference between a table-spoon or a tea-spoon, argggggg.

I'll just figure out for myself from trial and error I guess.

It's been a few days, and I checked yesterday, but the most diseased vine doesn't show any improvement. I re-sprayed some solution yesterday, and I'll maybe do it again on Wednesday.

I guess it doesn't help that some invisible enemies are eating little holes through the leaves as well. I guess I should do something about that.... but there's a big city ban on any sort of pesticides right now.

You shouldn't regain the affected leaves. Powdery mildew has been a huge porblem for me, but this year I seem to have conquerred it. It doesn't rain here, but after every time I wet the leaves I spray the whole vine with some sort of sulfur powder mixed in water, probably like terrybull said. It seems if there is any mildew on a leaf at all, you will lose it. Just make sure to get all the unaffected leaves while you can. I haven't heard of milk with baking soda, I always just use this powder. But at any garden store you should be able to get pre-mixed fungicide solution to prevent the mildew, but I go through it too fast so I mix it myself.

Well my lesson is learned, I should probably not have crowded my garden so much. I"ll be expanding it greatly for next year.

I wasn't expecting to cure the diseased leaves right away, but I know the bi-carbonate changes the pH of the leaves and makes it really unfriendly to the bacteria.

I guess next year I'll be spraying that stuff a lot more. I also did a lot of watering on the leaves this year, but was told it's probably better to just use a soaker-hose.

I didn't even know soaker hoses existed until a month ago. But now I have 150 feet of them on my pumpkin.