70+ of my plants have had that, after reading and searching and asking around found there are a few different things that cause the same symptom. Could be pests(esp. broadmites), virus, herbicide exposure or nutrient imbalance. I think I've narrowed mine down to nutrient imbalance. I've treated hard and heavy, top and bottoms of leaves with micronized wettable sulfur 3 times, just in case it's mites, but even with a 60x microscope couldn't determine if it was them or not. I trimmed all the affected leaves, and have been flushing my containers every few days and have on a drip system running 30 min/day. The new growth is starting to look normal. In my case I used some ferts that weren't idea, maybe released too much N too fast, from what I can deduce. Once I think it's flushed out, I'll add back in the ferts I used last year that worked (Osmocote). Hope that helps.