This looks more like blossom end rot although you are correct when saying that hornworms feed on tomatoes and not just leaves. If you haven't had a hornworm infestation the blossom end rot is likely. It is very common and often related to irregular moisture levels- going from very dry to a heavy watering, for instance. In some situations, low soil calcium, high pH and high phosphorus levels interfer with calcium absorption. Your photo looks like the area has been sealed off by the fruit. You can still enjoy this tomato; simply cut out the affected area. https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm
Answered on August 4, 2018
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