The Great Pumpkin Failure

I have wanted to grow pumpkins for years, but I have never wanted to give up the room in my beds that pumpkins or melons need to spread out. This year I decided to experiment with growing some pumpkins in a very large container at the base of my enormous Oak tree and let them grow around the stone retaining wall. I thought that if it worked, this would be visually beautiful as well as providing me with pumpkins for the Fall.

Well, this has turned out to be a miserable failure and now I am trying to figure out what went wrong. The vines started out growing very vigorously, reaching almost 12 feet in length after just a few weeks. There were lots of flowers and I was so excited when I saw tiny little pumpkins developing...



But gradually the leaves closest to the container started to yellow and wither, and most of the small pumpkins just shriveled up and fell off of the vine. I decided to radically prune back the vines to about five feet and I cut off all of the damaged leaves. This seemed to re-stimulate grow, and I got a whole new batch of flowers, but the yellowing leaf syndrome has continued...



Here is what the leaves look like when they start to fail. They get a powdery white all over them and then turn yellow and just wither...



So having absolutely no experience with Pumpkins, I am going to throw this one out there to any experts for some feedback. Does this look like a disease that is unrelated to the fact that I am trying to grow pumpkins in a container? Or do you think this is stress related? I have used some organic Fish Emulsion/Seaweed fertilizer a couple times, but I have not followed any specific feeding program for Pumpkins. I planted them in a Mushroom soil mix, the same soil I used in my Tomato beds.

I would be grateful for any thoughts or comments...

Editor's Note....

Wow! Within minutes, Colleen Vanderlinden advised that this is Powdery Mildew Fungus, and suggested a natural baking soda based spray to help fight it... Thanks Colleen!!

If you are seeing powdery-looking patches on the foliage of your plants, you most likely have a case of the very common powdery mildew fungal disease. Here is a simple spray for controlling the spread of the fungus. It won't get rid of the fungus on leaves that already have it, but it will prevent it from spreading to the rest of the plant.
Ingredients:

1 gallon of water
1 tablespoon of baking soda
1 tablespoon of vegetable oil
1 tablespoon of dishwashing liquid
Mix the ingredients together and add them to a spray bottle. Spray your plants weekly, preferably on overcast days to prevent it from burning the foliage.