I've never had this on any of my greenhouse or outdoor plants before

And if it's the only one now showing symptoms the first question is did you buy this plant or start it from seed and if from seed was it purchased or traded seed?

I'm no good on wee critters and it looks a bit like that's what's on the underside of the leaf shown, but the top of the leaf usually doesn't reflect aphid infestation or other infestations, maybe whiteflies?? and why one plant and not all the plants in the greenhouse unless the critters were introduced with that plant and just haven't spread yet?

Carolyn

Carolyn -- Thanks for responding. This plant was started from purchased seed (TGS, as I recall). All the plants in the greenhouse were started from seed. I've seen no evidence of either whiteflies or aphids. It does look like wee critters on the underside, mostly along the "veins" of the leaves. All the plants have been in the greenhouse since mid-May. This is very perplexing!

Sherry