If you see lesions like these on your tomatoes or potatoes you need to ACT QUICKLY to save them. It is LATE BLIGHT and is the same organism that caused the Irish Potato Famines. If you see it pull the leaves off and put in a plastic bag, seal it, and throw away. Do not put in a composter as it could overwinter there. If you don't do anything it will work its way through the stems, the tomato fruit, and can even go down into potato tubers. Depending on conditions it can take a plant down in a few days to a week or so. My tomatoes were drip irrigated so they only got wet when it rained but due to all the showers (especially at night) recently it has been infected.
I end up harvesting all the fruit and discarding the plants because it was impossible to stop. There is a series of three images that shows the same leaf with faint lesions, the more distinct only 10 hours later, then filling in more another 15 hours later. The day I first discovered it there were a dozen or so leaves showing symptoms and 24 hours later they were on nearly every leaf. The bottom picture shows it getting into the fruit.