kubali

I am no expert on lichen but I will say this, I have never known them to kill a tree or even hurt it in any way.so you might look for something else. What kind of tree are they. Looks like pear?Although the pear tree is an adaptable grower, some conditions are simply too hot or too cold for the pear tree. Warm winters can damage and, over time, kill a pear tree by preventing it from entering its dormant phase and leaving it weakened and open to attack from environmental and microbiological factors. Pear trees require between 41 and 50 consecutive days below 45 degrees Fahrenheit for an adequate dormant period. Grow a pear between U.S. Department of Agriculture hardiness zones 5 through 8 to prevent climate factors from killing your pear tree.