Not sure what it is or what to do?
A fellow Master Gardner, Steven Johnson, created an excellent chart summarizing 3 different tomato diseases: Early and Late Blight and Septoria Leaf Spot, including how to identify them. He also offers helpful organic and chemical treatments:
http://extension.psu.edu/plants/gardening/news/2015/early-and-late-tomato-blights-and-leaf-spot
