Hello Julia,

The symptoms pictured look like those of anthracnose, a very common fungal disease that can affect many kinds of shade trees, especially in years with wet spring weather. Fortunately, despite looking drastic, trees that are otherwise healthy recover well without intervention. (No fertilizer, no pesticides.) Diatomaceous earth isn't a plant supplement; it's used in some situations (like around young vegetable plants) as an insect or slug/snail deterrent, and it does not work once it gets wet. It will not have any impact on fungal diseases like this one. The leaves pictured also might have a separate, secondary leaf spot disease, but this too is not harmful and just an eyesore.

As we move into summer and drier weather patterns overall, any new growth the tree produces (such as if it sheds infected leaves and grows replacements) will be less likely to get infected, at least by anthracnose. As long as entire branches aren't dying back or remaining bare, the tree should be fine. If dieback is observed, have the tree evaluated by a certified arborist, since other issues unrelated to leaf infections would then be causing the tree stress or damage. Arborists can't always treat every condition, but they can help to make a diagnosis as well as look for sources of tree stress that might be able to be alleviated (like mulching too deeply, for example).

It's hard to say how long it will take the tree to look better due to new leaf growth, but it might be a few weeks. As an example, wild sycamore trees in our region were hit with high levels of anthracnose infection a couple springs ago, and by that summer they had leafed-out again and looked fine, and they also leafed-out normally this past spring. Plant diseases can be very weather-dependent, so springs with drier weather tend to have less widespread disease.

Miri