Fruit trees are always a favourite with people living on the prairies. Over the past 10 years I have seen an increased incidence of diseases on various fruit trees.
Earlier this year I discussed the many issues apple trees are having with fire blight and apple scab diseases. Apple trees are also infected with black rot disease, which I want to discuss in this article with reference to plum and cherry trees. Black rot is a fungal disease responsible for splitting the trunk (especially) longitudinally and forming a canker disease in that opening. Invariably this results in the death of the fruit tree. Formally the disease is called Botryosphaeria -- a broad class of fungal diseases that damage and kill many other trees in addition to those that bear fruit.
There may also be a grafting problem between the cherry/plum root stock and the main shoot where the union occurs. For example Evans is a common cultivated variety of cherry. The Evans shoot or scion is often attached to the root stock. Problems can happen as the tree gets larger, as the scion and root stock become incompatible. One of them may grow larger than the other. If a poor graft union occurs, then it is very likely that the trunk tissues will split, opening up the interior of the tree.
People often call me concerned about splits on their fruit trees and whether or not there is cause for alarm. The disease almost always enters the tree through wounds in the roots, or in the trunk and branch bark. People are the main culprit, but animals can also be the damaging agent as well. During the present year I have encountered a great deal of this disease in plum and cherry trees. As the disease progresses internally the bark can open right up revealing a large area of damaged wood. The action of the disease may cause an incompatibility issue with the now older scion and the root stock. The black rot fungus encounters the conducting cell tissues in the wood referred to as the phloem (living bark tissue) and the living xylem tissues. In advanced infections, droplets of diseased sap seeps from the wound where it appears to congeal in the air, leaving a gum deposit. Often this deposit has an amber colour. The physical manifestation of this disease is often when homeowners call me concerned about their fruit tree. The disease at this stage is referred to as gummosis. The gummosis part of the disease is illustrated for a cherry and plum tree.
During 2015 I have come across at least 15 cherry trees (all Evans variety) with the black rot disease; however, on the plum trees the disease can also show up with significant feeding of the fruit by the plum curculio insect. I could not find any references to connect the disease with the feeding by the insect, but transmission of diseases by insects to fruit trees is well known. Plum brown rot disease used to be common in the city; however, I have not seen this disease for over 10 years, although it used to be the most common disease of plum fruits.
There is a growing trend for homeowners to plant trees at home bearing edible fruit in Canada. Always be on the lookout for unusual minor bark splitting patterns in fruit trees. Early detection can encourage the property owner to seal up the splitting opening with tree wound paste or tar. Enjoy the fruits of your labour.
viburnumtrees@shaw.ca
