|This appears to be a "leaf spot" fungus on Gambel oak.|
Even though this article is from the Colorado Springs Gazette, it is probably a re-written Colorado State Forest Service news release, hence the northern Colorado focus (just another microaggression).
Some stands of aspen and cottonwood trees across northern Colorado and along the Front Range won’t be their most picturesque this fall, due to leaf spot diseases that benefited from an unusually wet spring and early summer, state foresters say.I have been seeing a browning of Gambel oak leaves in some clone-stands all summer, and since it could not have been from pesticide (not on our land), what was causing it?
Foresters say they’ve seen an unusually high degree of leaf blight in the mountains and along the Front Range – as far south as Aspen, the Collegiate Peaks and Colorado Springs – for about a month.
At least two fungal diseases are to blame for the leaves now showing significant spotting or dark splotches. Marssonina leaf spot is caused by the Marssonina fungus and is the most common leaf disease of aspen and cottonwoods in Colorado. The disease can be identified by the presence of dark brown spots or flecks on leaves, which can then fuse into large, black splotches on severely infected leaves.
Apparently the fungus affecting oaks is different, Discula quercina (and maybe others), but the look is the same: "Leaves have scattered brown, irregular spots that can coalesce into nearly completely brown leaves." And the extremely wet spring is to blame.