Anchracnose defoliates sycamore trees annually in our area. There are a couple of things you can do to mitigate the problem. The best thing you can do is wait. The tree will likely put out a second flush of leaves. In the meantime, the damage is mostly cosmetic.

In addition, it is important for you to rake up and destroy (do not compost) the fallen, infected leaves. This will remove a lot of the fungal spores that can move about by wind or rainfall and also over-winter on the ground. 

Spraying with fungicides are generally not called for unless the tree is small and not well established. The information in the links below will be helpful:

https://www.canr.msu.edu/news/sycamore-anthracnose-why-your-sycamore-may-be-slow-to-leaf-out-this-spring

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/fungal-spots/sycamore-anthracnose