London plane tree with yellowing leaves
It looks like it could be damage from the feeding of a Sycamore plant bug (FYI London Plane Tree is a Sycamore). Take a look at this fact sheet from Utah Extension https://utahpests.usu.edu/ipm/ornamental-pest-guide/arthropods/plant-lace-seed-bugs/sycamore-plant-b...
Two thoughts: 1) in your photo, I can see rock mulch in the background. If the sycamore is growing in an area mulched with rock, improving the growing conditions might help. For instance, rock mulch create a hot and, therefore, stressful environment that could be conducive to the pests so you might consider swapping out the rock for wood mulch or even planting plants instead. How is the tree watered? You should be watering the entire root area of the tree. 2) Try spraying your tree in the dormant season with a horticultural oil to smother the insects that are overwintering on the tree. If the tree is large, you may need to hire a licensed tree company to do it to ensure good coverage. Here's some information about horticultural oils: http://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/
Thanks for the answer. I have checked the tree quite often and never seen bugs on it, nor any that look like sycamore plant bugs in the pictures on the internet. Are they fairly hard to find or detect?
They are only a 1/4 inch and, according to the Utah fact sheet, 'present until mid-summer,' so they might not be present on your tree now. In any event, since this has been an ongoing problem for the tree, it wouldn't hurt to get an arborist out to do an evaluation. Someone on-site is going to be able to see a lot of things that we can't see in a photo.
Denver Forestry has a list of licensed contractors on their website or you can use the 'find an arborist' tool on the website of the International Society of Arboriculture (https://www.isa-arbor.com/) to find an arborist.