Garden Pests and Diseases: What damaged my begonia stems?, 0 by LAS14

LAS14 wrote:
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