Q: I found this nasty disease on one of my hosta plants – apparently petiole rot. It seems odd to have this in summer drought conditions and (so far) only on one plant out of many, many hosta in my yard. I’ve watered by hand, just enough to keep things alive .
A: Wow – great pictures! Your diagnosis is correct: petiole rot, otherwise known as crown rot or Southern blight of hosta.
The little beige balls and the cottony webbing is definitive for Southern blight.
I’m curious too how it could occur when you water so little. The disease is always associated with excess moisture in the soil. My best guess is that you watered with a strong stream of water and it bruised the stems, setting them up for infection.
Followup: I confess to sometimes using a ‘strong straight stream’ as I thought it harmless and a time saver. Never knew it was a risk. The plant is one of about 13 divisions of what was a huge plant. None of the others have problems. I checked on nearby plants tonight and saw no other problems.
Ah-ha! Case closed!