Two weeks ago I have planted a few fruit trees in my garden. The plum tree has holes eaten through them between the leaf veins, leaving them looking rather lacy. I haven't seen any actual insects, but am worried about the damage to this young tree. The apple leaves are curled upwards. Two sweet cherry trees and the peach tree are having yellow leaves with dark spots which are falling down. I see often a very small bright insect flying around.
The holes in the leaves of the plum tree sound as though some type of chewing insect or worm must have attacked them. Whatever it is/was, it's likely gone by now. Unless you're seeing additional defoliation, there's no need to spray. Fortunately, trees can withstand a great deal of defoliation with little consequence. Yellowing leaves with dark spots sounds more like a fungal disease problem. Again, the infection period is likely over. It's important to use preventative fungicide sprays to prevent diseases on fruit trees. Contact Cornell Cooperative Extension in your area for their current fruit tree spray recommendations.