Several leaves on our Redgold nectarine have developed these pale, wart-like clusters this year here in upstate NY (zone 5B). We’ve been plucking the leaves as we see them, and it seems to be pretty well under control for now, but we’d like to know what we’re dealing with.

Is it insect damage or bacterial/fungal? And is it something serious that requires spraying, or just a cosmetic issue?