I'm having a problem with my pumpkin leaves turning yellow. There are all these white spots or residue on the leaves and some black spots on the back. It seems to be spreading to my squash an zucchini plants as well. The attached photo is from my zucchini. The pumpkin looks the worst an it is more than 2/3 yellow an wilted. The squash an zucchini only have a few leaves that are yellow but almost all have the white residue/spots on them. The squash and zucchini have been producing well and have lots of new growth. However, I only have one pumpkin on the vine. I am not sure if i am over watering or under watering. I came home this afternoon and watered the pumpkin and within 2 hours it looked not as wilted.This is the first year I have grown a garden so any help is greatly appreciated.