White powdery surface on leaves can indicate powdery mildew, which is indeed a fungal infection. Once it appears, there is nothing to stop it for this year. Be sure to remove all the leaf debris when the leaves fall and clean up the area very carefully. The plant looks like it's in a wide-open space (thank you for the range of images!), so it should have good air circulation, but it is also very dense. So you might consider some thinning next year to ensure air moves through the tree well. The tree is too large to spray with a fungicide, but if it were smaller a regular spray of a fungicide next year before  symptoms appear would help. However, these things are often related to weather in a particular season, and with different wind and rain patterns next year may not be a problem.
For this year, don't worry about it. It is late in the season and any leaf loss will not harm the plant at this point. Just clean up really well in the fall. It'll be fine!