Hmm. It's the time of year when plants are burning from higher light levels. Have you moved the plant into more sun recently?

Michael
Interlaken, NY Z6

Michael, no the Sans usually stay inside....I grow them under lights. They are fairly bright fluorescent lights, but I wouldn't think they'd be bright enough to harm young foliage, would they?? I could move it from being directly underneath, if that might help.

Depends on the strength of light, 2x33W cfl in a homemade reflector will nuke any plant I've tryied at a distant of 1 inch from the plant, 3 inches away and sans are fine.

My first thought was sunburn as well, it's awful, distinct...
Has it possibly been treated with a spray?
Or maybe an over zealous squirt of windex...
It looks burned...

I get the same scaring with too much Sun. Your lights may still be too strong.
Stush