It looks like powdery mildew. Have you been watering from overhead ? Usually watering from overhead + humidity is almost a guarantee of mildew but there are plenty of readily available fungicides that can be sprayed on the leaves (so readily available even Target carries them). Also, you have them in containers ? You might want to re-think that and root them in the ground. Pumpkins need alot of space and nutrients and depending on the size of the container, it may not be big enough. Good luck !!

Thanks, misswissa, that is very helpful! I'm not sure what watering from overhead is (I am always careful to not to water the leaves but rather the base of the plant) but it has been exceptionally humid and hot the last few weeks here. We've had a lot of rain.

I bought some fungicide and treated yesterday, hopefully that will help. Yes, they are in (very large) containers...I wish wish wish I had a ground garden to put them in but sadly I do not. I figured it would be an experiment to do containers, even for mini-pumpkins...an experiment that may have failed already thanks to the powdery mildew, but I guess we'll see! One of my healthier plants has a flower that opened up just this morning...crossing my fingers!

Hi there from Michigan - as you plant the seeds from now on, try soaking them in hydrogen peroxide and water - equal parts of each - and then as they grow spray them (and any other plants that get powdery mildew - I just spray them all!) with a mixture of 1 part skim milk and 9 parts water - you can add a couple of drops of dishwashing detergent to that if you really want to. Spray away. I have only had PM on my plants that I forgot to spray in the spring!!! Go to google and do a search for "powdery mildew milk" (don't use the quotes tho) and see what comes up.
Magic in Michigan