You don’t say what kind of tree this is but my guess is it’s a crab apple.

It looks like you have some sort of leaf spot disease but I can’t diagnose it from your photos. I don’t think it is Fireblight - but again impossible to diagnose from the photos. Below is some information that should help you.

Fungicides are ineffective unless administered prior to see the damage. 

We have had 3 years of drought and ll of our trees are stressed. While we have had good rain this spring, be sure to keep your tree well watered through the summer when it gets hot. Stressed trees succumb to problems easier than healthy trees. You have a very large mature tree and from your photos the damage does not appear to be extensive but you will be able to see that Bette than I can. If you feel the damage is extensive, you should call an arborist to come in person and look at it.

Be sure to do leaf clean up.

https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#causes-of-leaf-spots-1156260

https://extension.umn.edu/plant-diseases/fire-blight#leaf-symptoms-1766160