I grow over a dozen tuberous begonias in pots every year. This year one and only one suffered severe damage around both stems. There is a copper ring around the plant, and it may be tempting to think the plant rubbed on the ring, but all of my plants have the same kind of ring (for slugs) and the damaged one was not touching the ring. I'll post a closeup of an undamaged plant and my whole patch just to illustrate that.
TIA
LAS
Here's an undamaged one with a collar.
Here's a picture of all the begonias with copper collars that didn't have a problem. The damaged one is on the lawn at the very front.
Were any of the stems/leaves on the damaged one dangling down low enough that they touched the grass? The other thought is that the surface of the copper oxidizes over time, and if that oxidized layer is thick enough then it won't be effective as a barrier anymore. From the picture, it doesn't look like your copper is shiny so for sure it's got some oxidation, but it's hard to tell if it's enough that it would cause a problem or not.
The copper is still shiny and no leaves were touching the ground. Even if something (like a slug) could crawl in, what would cause that kind of damage? Anything could crawl in except a slug, I assume, as the copper interacts with their slime.
LAS
That looks light begonia stem rot.
http://plant-disease.ippc.orst.edu/factsheet.cfm?RecordID=153&rec_type=disease
I volunteer as a MG in a botanical garden greenhouse that ( among 100's of other plants, trees and shrubs) propagates its own begonias. We see it from time to time. It isn't unusual for nearby plants, even in crowded greenhouse conditions, to be unaffected. The affected begonias are tossed in the trash pot and all - not composted.
Thanks, Snapple45!!! That really looks like it must be it!
LAS
