Q: I planted a large bed of pansies in mid- October. Most have been growing well. However, I noticed one plant that appears extremely wilted. Upon examination, it seems the stem of the plant at soil level narrows to the size of a thread — almost as if it had been chewed, though the stem was not totally severed.

Can you tell me what’s going on?

A: The stem “narrowing to a thread” is a classic symptom of crown rot. It affects many plants but it is most often associated with excess moisture in the soil.

It is my observation that garden centers water their pansies excessively. The sick plant may have been infected when you bought it.

If only one plant is affected, remove it, plus the soil around it, and plant another.

see
http://www.aces.edu/pubs/docs/A/ANR-1214/ANR-1214.pdf