Ditto, nailed it Patihum!
We get grey mold (Botrytis) here and some Alternaria (early blight) - and on the stems, grey mold is worse in my experience. the Alternaria stem lesions, with the concentric rings, tend to be shallower and the plants can survive for quite some time - even months if not badly affected. The grey mold on stems will usually eat through and kill the stem within a week. That is in a greenhouse environment where the plants are protected from rain but get high humidity.