Rainy summer weather brings high humidity, which favors the development of leaf spot diseases.
I commonly see the diseases affect sweetgum, poplar and maples in summer. Not all trees will be infested…even two trees standing side by side may be affected differently.
The spots are caused by various fungi. On sweetgum, it may be sweetgum anthracnose, Gloeosporium nervisequum, or sweetgum leaf spot, Cercospora liquidambaris.
Cercospora leaf spot is common on maple and poplar.
The leaf spots do not usually cause great harm to the trees. They can be completely defoliated one year and come back healthily the next.
If you want to do something about the diseases, rake up all fallen leaves in November and replace them with clean mulch under the tree.
If they REALLY bother you, spray the trees with Mancozeb, Kop R Spray or products containing chlorothalonil when spots are first seen each year.