Trees/young English Oak
Not positive this is in your realm, but i'll give it a try..
We are in Houston and have an approximately ~2 month transplanted English Oak in a 3.5 year old house's yard (sand/clay under thin top soil from sod). It was the model home and has a sprinkler system.
Bought the tree in a 45 gallon pot, about 12ft+ high. It had no leaves, but was just budding all over. Once planted it took off and filled with leaves quickly. Now my wife noticed it has random, but evenly distributed yellow leaves over the whole tree. Our sprinkler was set to 4 time a week over night for about 2-5 minutes depending on grass or flower bed. I recently cut it to 3x a week but have been doing some manual watering of flowerbeds where we got rid of some excessive flower beds and put new sod. And I can't say when the yellowing started.
When I cut the water, or when I then added more for the sod is up in the air (The tree has a small bed around it with an herb).
Ideas? With some simple googling I've seen over watering or possible insect. We are at the edge of the neighborhood with some thick trees/brush area across a retention pond.
If you could attach a picture to the question--that may help ID the problem. I would try fertilizing the tree with 10-10-10 fertilizer at the rate of 1 lb. of fertilizer per inch of trunk diameter scattered around the tree and watered in good. Apply the fertilizer just before a rain event and you will not need to water. Do not use a product called Weed and Feed in the lawn near the tree--this contains a herbicide that will damage or kill the tree. Watering with the sprinkler system should not be a problem unless the water in pooling up around the tree. Sprinklers usually putout about a 10 of an inch of water and that is not enough to cause a problem with trees., Sometimes if the soil is a heavy clay when the hole is dug for the tree especially with a auger the sides of the hole are smooth and makes the hole have the characteristics of a "pot" in the ground. This will then hold water and drown the roots. Check the soil next to the trunk and see if it is wet. This can happen if the hole was not dug large enough for the root ball. The planting hole should be 2 times the width of the root ball and filed with good top soil. With sand/clay soil this should not be a problem. I would try the fertilizer. Without seeing the tree this would be my thinking.