Rose of Sharon in distress
I have a mature (25 yrs) Rose of Sharon that has developed a yellowish powdery growth covering bark on main branches that is killing off or severely limiting leaf growth on that effected section. It is not on leaves only the bark. Picture is attached. Any treatment suggestions?
Monroe County New York lichen
The yellow growth you are seeing on your rose of Sharon is lichen and is not harmful to the plant. Rather, it suggests humidity and slow growth. An older shrub growing slowly is natural and not necessarily a reason for alarm. The lichen is not corrupting the plant on its own. But while the lichen is not causing a problem, it may be a signal that your shrubs are in distress from another source.
We have seen a lot of lichen appearing on trees and shrubs in our area this year as it has been unusually moist in the early months of this year as well as last. It also is possible that your plant is in too protected a position where moisture is accumulating and that any thinning of nearby plants that you can do to increase air flow will have a benefit.
I do see newer growth on your plant but if you are seeing dying twigs or leaf loss out of the ordinary, then consider whether your shrubs are experiencing root problems from drought or excessive moisture. Adding compost to the soil to improve the texture and drainage would be wise if you see other signs of distress. Otherwise, there is nothing to worry about but note that it is an aging plant.