Hi Dane, Thank you for your inquiry. It appears that your oak tree is infected with a fungal leaf spot disease. Generally these diseases, although unsightly, are a minor stress on the tree, and fungicides are not needed...unless the tree is defoliated for two or more years. In addition, fungicides will not "cure" leaves that are already infected; but they help prevent the spread of the disease to other parts of the tree. As far as the disease has progressed, applying fungal sprays would be a waste of time and money at this point.

Fungal diseases develop and are most common during periods of rainy weather, and we have certainly had a lot of rain so far this summer. During the growing season, fungal spores are transmitted throughout the canopy of a tree and also throughout a neighborhood to susceptible plant tissue by wind and splashing rain. This would explain why nearly every tree in your area is affected.

Some helpful actions to take are listed on the link below. One of the most important ones would be rake up and destroy the fallen leaves as the fungal spores can overwinter on those leaves left on the ground and re-infect the trees next year.

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#causes-of-leaf-spots-1156260