I don't think the cankers are being caused by a lichen, although there are lichen in the picture. They are the gray patches, while the orange rings look like a perennial fungal canker. I won't venture a guess on exactly which one.
Most, if not all, Lichen are not harmful to trees. They tend to be an indication of fairly good air quality and have little or no effect on the tree. Lichens are an unusual creature, kind of half fungus, half algae. And there are a lot of different species, with comparatively little research done in regards to tree health.
Fungal Cankers do affect tree health. Small cankers on twigs only affect the tree from the point of the canker to the tip. It's when they advance to larger stems and trunks that they can be a major tree health problem.
Cutting them out can reduce the spread by eliminating reproductive spores. Clean your tools with a 10% bleach solution or alcohol between cuts, to prevent spreading spores to wounds created by pruning. Then a shot of pruning paint would help prevent spores from landing on the wounds until it hardens off.
There are sprays that can be use in bad cases, on high value trees, but I have no personal experience using them, so I'll default to those that may have.
It also helps to improve growing conditions for the trees. Cankers often attack stressed trees. Develop a chip bed out to the drip-line of the trees. Remove the grass by weed-eating down to the soil, don't disturb the soil! Then add compost, and cover with a thin layer of decorative chips. Total depth of compost and chips should not exceed 2" or 3". If weeds or grass do come through the mulch, a small spot treatment of roundup will work, or better yet just pull it out by hand and toss it on top of the chips to decay into tree food.
Water during periods of drought.
http://www.forestpathology.org/canker.html#nectria
