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If you see irregular brown patches (sometimes grayish brown) on leaves over large parts of your oak or maple and are similar to the ones in the photos to the side then odds are it is one of several fungi collectively called anthracnose.  Anthracnose shows up in Oak and Maple by late June well into early September. Actually Anthracnose can occur on a variety of deciduous trees but Oak, Maple  and Sycamore are the most common (it used to be common on Ash too but thanks to Emerald Ash Borer there aren't too many ash left to mention it)

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 Anthracnose generally doesn't hurt the tree but can be unsightly and will cause some leaf drop.  Some literature suggests raking and removing leaves to limit infection however wet, cool summers (or humid) are the main driver that allows the disease to show up.  Keeping your tree healthy is the best strategy. 

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 Anthracnose can be confused with damage to leaves by heat/drought called leaf scorch. The symptoms are very similar but the tell tale is  leaf scorch always starts at the leaf margins (edges) and moves inward and its usually pretty obvious that the tree is otherwise under drought or heat stress (because of hot parking lots, really dry dead grass underneath the tree, extreme drought...) 

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Anthracnose on  London Plane Tree/Sycamore  can be scary.   Sycamore  usually gets anthracnose in mid spring just after leaf out and will suddenly drop nearly all of its leaves.  Do not panic.  The leaf drop is normal for sycamore with anthracnose  and because the leaf drop is so early, the sycamore will produce a whole new set of leaves within a few weeks and usually will keep them this time as the time for anthracnose to spread on sycamore has passed.  It is very rare that anthracnose will impact a Sycamore enough to put it into decline.