Spider Mites

This appears to be the damage done by spider mites, a common problem. Symptoms of injury include flecking, discoloration (bronzing) and scorching of leaves. Injury can lead to leaf color loss and even plant death. Natural enemies include small lady beetles, predatory mites, minute pirate bugs, big-eyed bugs and predatory thrips, but of course that’s only outside. One reason that spider mites become a problem is insecticides that kill their natural predators. Another is low humidity, such as indoors in winter.
Spider mites are barely visible to the naked eye, and are difficult to control. Insecticidal soap can help, especially if the problem is caught early and the leaves, both sides, are sprayed well. Or look for another product with “spider mites” listed on the label.