Why Is my Tree Yellow?
It's July, but your tree looks like it's headed into late October. This yellowing of the leaves is called "chlorosis" and is most frequently caused by an iron deficiency. The best way to identify chlorosis is to inspect a leaf. If the tree is chloritic, the leaf will be yellow, but the veins in the leaf will still be green.
Iron injections are frequently prescribed by tree services and nurseries, but most of the time it won't help your tree...
Close up of interveinal chlorosis
How Chlorosis Affects Trees
1) Aesthetics. Many trees, like autumn blaze maples, are planted for their vibrant color. If the tree suffers from an iron deficiency, those desirable colors won't develop.
2) Nutrition. Tree leaves are supposed to be green because they are supposed to have chlorophyll, the pigment that enables photosynthesis to take place. Without sufficient chlorophyll, the tree cannot make the sugars it needs to thrive.
Causes of Chlorosis and Treatments
1) Iron deficiency. If the soil lacks iron, then adding iron to the soil can do the trick. However, much of the time in Utah's valley, the soil has iron but the tree cannot get it. If that is the case, adding iron to the soil does not help. You can get a soil test to measure the iron content of your soil.
2) Soil pH. Most landscape trees that get planted in our area are native to somewhere else, somewhere that has acidic soils. Our soils have a relatively high pH. Higher pH causes iron in the soil to turn into solids, rendering it unavailable to be used by roots. The pH of your soil can be determined through a soil test. If your soil is basic and needs acidification, you can apply sphagnum peat, elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, or organic mulches. Clemson Cooperative Extension has a nice table for determining how much sulfur to add to obtain the desired pH.
3) Reduced airflow. Compacted soil, waterlogged roots, plastic sheet mulching, and stripped top-soil aggravates chlorosis. Avoid overwatering and use organic mulches.
4) Pathogens. Some fungal and bacterial infections can cause chlorosis. This would need to be treated with proper identification of the pathogen and appropriate pesticide or fungicide application. Be careful though, because some pesticides and herbicides can also cause chlorosis.