Update: Thanks for all the suggestions. All I can report is that as unexpectedly as it began, the strange growth habit has stopped. After three months, they've gone back to normal. The babies are growing out perfectly normally. As long as it doesn't recur, I'm contented to put it down to sunspots :)

Critter in the soil -- very fuzzy image, but by the shape I'd say pupal cases of moths. Non-feeding stage. To find out who they are, place in a covered clear container. Place where you'll see it daily but NOT in the sun. Eventually, moth will emerge.

Thanks farkee, I believe that may be what I saw. When I looked at the leaf without the loup I could't see it and there was only one on each leaf that I examined. I'll look closer tomorrow. Thanks for the education.

It could be something called silvering (which is common in EU greenhouse production) It could also be evidence of some kind of physiological damage from a chemical (another issue with being in a poly tunnel). A number of things could be causing that issue such as exhaust from a poorly performing heater, off gassing of the plastics or some kind of pollution. It could also just be a "burn" from some quick exposure to a change in temp or light. You might try looking at the Univ Mississippi or Calagary info for greenhouse growers.