Attack of fungi. May be due to high humidity.
I assume that the tree will survive with no major problems.
When leaves will fall they need to be destroyed. Do not compost.
Eventually treatment with a fungicide. But it is difficult to a large tree. Because must be bathed completely.
I have no experience in America with treatment. Perhaps with a 3 in 1 solution based on Neem extract.
If it is rain the treatment is useless.
But it may not need treatment. If is a rainy weather very likely the sun will do the treatment. When time will be nice with sunshine.
Generally I avoid, as much as possible, to do treatments unless absolutely necessary.
