I think that the spots are likely caused by insects feeding on the leaves such as thrips. Thrips are tiny and hard to spot but if the infestation is bad then you should be able to confirm by carefully searching the plants, turning the leaves over to look for the pest. You could also try gently shaking the plants while holding a piece of white paper under it to see if anything drops out. Any badly infested leaves can be removed. Failing that, an insecticidal soap should take care of the problem.