Many trees and shrubs are finishing up top growth for the season and entering leaf senescence early to escape this long, hot and dry growing season. Tar spot lesions are becoming apparent on maple, most notably Norway maple, at this time. The lesions are about 15 to 20 mm in diameter, yellow, then turning black with the formation of fruiting structures. Chemical control is ineffective at this time since infection took place early this spring, when the leaves were emerging. Where infestations are significant, rake up fallen leaves and destroy them to reduce disease inoculum next spring.
Lesions are small, circular, rusty orange spots on the tops and bottoms of leaves.
These were infected earlier this spring by spores from the alternate host, juniper. Look for finger-like structures at the bottoms of the leaves as these will be releasing the spores which will infect juniper. Powdery mildew is becoming prevalent on broadleaf ornamentals at this time. It appears as a white to greyish powder on the tops of leaves. It thrives during hot days and cool nights so we can expect this disease to progress in August.