Without knowing what sort of tree it is, or where you live, it’s kind of hard to say. I’d pretty much discount fungal diseases, only because you say it’s “eating away” at the leaves and most fungi cause leaves to spot or yellow and then drop. Yes, there are tatter-leaf fungi that cause a leaf to look eaten, but powdery mildew isn’t one of them.
Caterpillers? Maybe. It’s kind of late in the northern zones for them, but you might be down south. And I did see a few green worms munching on some ornamental kale today, so it very well might still be an insect larvae of some form or another.
Aphids? Possibly, although they don’t really cause an eaten effect, but more of a shriveled leaf effect as they drain the juices out of the leaves. But as the honeydew on their back molds, they can definately look white, small, and fuzzy.
Mealy bugs? Scale? Again, it’s possible and they fit the description, but like aphids, they cause more sucking damage than chewing damage.
If it’s chewing damage, with little discoloration outside the chewed area, figure it’s most likely caterpillers or beetles. Most likely, spray with Diazinon which is a little like Raid for gardens (damn near all purpose), but do make sure you know what it is first and read the label.
If it’s a shriveled look with masses of white things under the leaves, you have a sucking insect such as scale, aphids, mealy bugs, etc. I’d say spray with Malathion which seems to work well for me on sucking insects and is slightly less potent than Diazinon.
If it’s a fruit tree that you plan to harvest from, do not use either of the above. Strongest chemical I’d recommend would be a carbyl based spray like Sevin.
If the leaves are yellowing or browning out in spots under the fuzziness and eventually the dead parts are breaking away or the entire leaf is yellowing out and dropping, get a good fungicide such as Daconil.
Or… (this is a long response, no?) figure your tree has a month left before autumn and all the leaves are going to fall off anyway and let nature take its course
“I guess it is possible for one person to make a difference, although most of the time they probably shouldn’t.”