We recently received a sample in the Plant Disease Clinic of a black walnut twig. The owner was inquiring about small holes found in the bark of the young branches. Rather than indicating a disease or insect problem, the holes are part of the natural anatomy of the twig. All trees have small pores called lenticels scattered over their bark, although they are more noticeable on some trees than on others. Lenticels serve as "breathing holes", allowing oxygen to enter the living cells of the bark tissue. Lenticels can also be seen on fruits such as apples and pears, appearing as small brown specks on the skin surface.
Since they are holes in the outer protective layer of the branch, lenticels can sometimes be invaded by disease-causing organisms, although this is rather rare. For example, the bacterium that causes fire blight, a common bacterial disease of apples and pears, sometimes enters its host through lenticels on young shoots. Some fungal and bacterial canker-causing organisms may begin the disease process by entering the plant through lenticels. Most of the time, however, lenticels simply allow the bark of a tree to "breathe".
Lenticels in the bark of a black walnut twig.
This article originally appeared in the 4/20/2005 issue.