Tiny yellow spots on leaves: pests or disease?

Yellow stippling like this is often caused by spider mites.

Examine the bottoms of the leaves with a strong magnifying glass.

There are several diseases which is happening in plants but yellowing leaves can be caused by a number of conditions. If the cause is obvious, you can fix it immediately. If the cause isn't so obvious, try changing one thing at a time until you fix it. Even after you correct the problem, however, it's likely that yellow leaves will fall off with time. Don't worry: if the plant regains its health, it's very possible that new leaves will fill in during the next growing season

Thank you very much for your replies! Looks like spider mite. I'll try the Scotts Bug Clear Ultra Gun" insecticide to get rid of them.