PlantLife Online

Answer

The white leaves on your pumpkin plants is most likely caused by Powdery Mildew. Powdery mildew is a fungus that attacks many different species of plants some more susceptible than others. Powdery mildew first appears as white, powdery spots that may form on both surfaces of leaves, on shoots, and sometimes on flowers and fruit.
The best management for powdery mildew is to plant disease resistant varieties when possible. Members of the Cucurbits family which includes pumpkins often need to be treated with fungicide. To reduce the chance of powdery mildew affecting your garden be sure to plant in sunny locations, provide good air circulation, and avoid applying excess fertilizer.

As for your tomatoes are the leaves or fruit turning brown? If you are describing the fruit my first thought would be a bacterial canker without seeing the tomatoes, however there a few other pathogens that could be the cause. Check out this link for a list and description of common tomato problems.

Thanks for the gardening question!

Plant Life Online