I do not recognize this so I Googled it. This might be helpful https://www.google.com/search?q=black+spots+citrus+leaves&biw=1680&bih=857&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjYltv_39nQAhXCRSYKHXBhDDIQ_AUIBigB#imgrc=3GynaX0KoECnTM%3A

Hmm...that looks like what I thought was black citrus aphids on my Ponkan which I wrote about in another post. It is strange how it keeps going after that one tree. I keep spraying and cleaning it off with oil spray or soapy spray but it keeps reappearing. I hope it does not spread to my other trees. The new leaves are beginning to harden up on the tree now so I hope the bug will soon give up on it, but also am afraid it will start looking at the new growrh on my other trees. I treat them all with the sprays. I can't figure out where they are coming from as I don't see any ants nor what about it that attracts them to that particular tree. I have another Ponkan at the other end of the greenhouse and so far that is not attracting them. I think maybe that is because that tree gets a lot of air movement as it is nearer a fan. I keep suspecting my hibiscus may be the host as I usually keep them away from my other plants as they are notorious bug magnets, but even they seem bug free so far and not near this particular tree. Maybe they are in the soil of that particular tree, but I have even drenched the soil and I am still getting the black aphid or black fly back for now about 2 months. It is interesting to me that it likes that particular tree. Cory

They can develop powdery mildew in late season. And slugs are a major problem here as are deer :-) On occasion, flower buds can also be damaged by earwigs.

I have a couple cuc leaves that look like that too......but only a couple, so I guess I won't worry about it....yet. Earlier in the season, I saw some white flies around the area. Don't know if they do that.