Did you spray it with something recently and then put it out in the sun?

I tried rubbing it off, but it wouldn't. It's on the top of the leaf. I did as you suggested, John, and removed it. When I did, I noticed barely visible beginning spots on the neighboring leaf. Hope I don't have a problem starting on this beautiful fruiting tree.

No, I haven't sprayed it recently, Laura. Not that I recall. This tree is outside in the sun most days.

Thanks for your quick replies.

More spots have appeared. It seems be coming on only the new growth leaves. I have no idea what these mean. I've been fertilizing with Foliage pro over the winter, and the tree has been outside in the winter during the day. Maybe I splashed some on the young leaves when it was cold, and this is the result? Or perhaps it is a disease of some kind? Sure would like to know what it is so I can take care of this little tree.

This is my mandarinquat tree. Thanks for any help/suggestions....

round spots generally indicate fungus; a simple fungicide spray will handle it if it is.

Thanks, John. I will take that approach to it and see how it goes.