A common disease especially under humid conditions caused by the ubiquitous fungusBotrytis cinerea. It is able to infect many different plants and typically starts on dead tissue.
Cell pack of affected tomato seedlings.
Stem canker (lesion) due to gray mold. (Next four images.)
Note the dead leaf attached to the stem in the center of the canker. The pathogen attacked the dead leaf first, then was able to move into the stem.
Close-ups showing sporulation. (Next two images.)
The gray fuzzy growth of the gray mold pathogen distinguishes this from other diseases, in particular late blight, and from secondary fungi that decompose dead tissue. A good hand lens or microscope is needed to see the growth in enough detail to confirm its identity.
More gray mold images