Question
I have two young trees on the east side of the front lawn.  One has its leaves turning yellow and falling and the other looks ok but needs watering.  Both have a green moss-like substance growing around the trucks.  

How do I treat this condition?


Answer
This sounds like lichens.  Lichens are unusual creatures. A lichen is not a single organism the way most other living things are, but rather it is a combination of two organisms which live together intimately. Most of the lichen is composed of fungal filaments, but living among the filaments are algal cells, usually from a green alga Generally lichens do not cause real health problems with trees but can signal the tree is under stress. I would water every third day fro a couple of weeks and then water every week if it does not rain. Water with 1 inch of water -place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop.
The lichens can be controlled by spraying the trunk with a copper fungicide--this will kill the fungi and the lichen will die. The main problem seems to be not enough water. If you have not done so mulch around the trees with 3 inches deep of organic mulch not piled up on the trunk. This will help hold the moisture on the roots.