A couple of my tomato plants have wilted. I am a successful gardener mostly by pure luck so I really don't know why I am having problems.
My neighbors are having the same problem with their tomato plants. The only thing I can think of is that all this rain is causing some sort of fungus but I don't see anything on the plant.
Checking a website from the Extension in Lancaster County, http://lancaster.unl.edu/hort/articles/2002/tomatowilt.shtml, the possible causes are lack of water, vascular wilts, tomato spotted wilt virus, walnut toxicity, or stalk borers. Most of these problems don't have a solution.
My neighbor is spraying her plants with Neem, a botanical insecticide that has a low degree of toxicity and is considered almost nontoxic to humans and animals. It is also completely biodegradable.
I have walnut trees in my yard but for years this hasn't been a problem, although maybe this is going to be a problem this year. Last summer, with the drought, the squirrels buried their walnuts in my garden because the ground in the rest of the yard was hard. I have been pulling out tiny walnut trees growing up from the buried nuts.
I probably am going to have to pull out the affected plants and hope that I can keep the problem from spreading to my other plants.