This is Powdery Mildew found on one of the squash plants in our garden. Please see the Blog post on April 10, 2012 for more information.
Tomatoes with Early Blight disease have a fungal disease - the tomatoes are unaffected and OK to consume but all of the diseased and brown leaves should be removed from the garden ASAP. There is no cure and you should remove the entire plant whenever it finishes producing tomatoes. Summer solarization will help kill the fungus - contact a Director if you're interested in solarizing your bed.