From February until early April, fruit trees grown in Missouri often become infected with a fungus (Leucostoma cinctum) that causes perennial canker. This fungus invades the bark during winter when temperatures are above freezing but stops when there is active tree growth in the spring. After infection, cankers with amber ooze become visible and can develop year-round (Figure 1). This pathogen is a secondary invader, requiring a wound for infection. Some of the common ways the pathogen enters trees is through wounds, such as bud scars or from twig lesions caused by brown rot, cracks in the bark from sunscald or Southwest injury, feeding sites from Oriental fruit moth, borers or rodents, and hail or mechanical injury. Once the tree is infected, spores are often found on the bark surface year-round. Young trees are most likely to succumb to perennial canker and may take three or more years to die.
Since wounds are often unavoidable on fruit trees, the best way to guard against spread of the disease is to select cold hardy cultivars for planting and maintain tree health with cultural practices. Careful attention to tree nutrition, especially preventing nitrogen and potassium deficiencies, will help keep trees healthy. Submitting leaf samples for foliar analysis from July 15 to August 1 will help determine the tree nutrient status and the amount of fertilizer needed to maintain productive fruit trees. Late winter pruning is also recommended to prevent premature deacclimation of trees. If possible, wait until just before buds begin to grow to prune. When pruning, avoid horizontal or flat cuts where water can collect and prolong infection periods. Also, prune healthy trees first and leave the infected ones until last to avoid spreading the disease. When pruning infected trees, disinfect the blades of the pruning tool between cuts. Household bleach (1 part bleach + 3 parts water), Pine-Sol cleaner (1 part cleaner + 3 parts water), or rubbing alcohol (1 part 70% isopropyl alcohol + 1 part water) can be used as disinfectants. Remove prunings from the area as soon as possible as the fungus lives and grows on dead tissues and will spread under rainy or windy conditions.
During the growing season, avoid trunk wetting when watering fruit trees. Also, tree trunks can be painted with latex paint to prevent bark splitting, especially on the southwest side of the trees. Maintaining a weed-free area or cover crop at a low height during the growing season and in winter will also help reduce injury from rodent feeding. Wire tree guards around the base of the tree trunks year-round or plastic wrap-around tree guards placed on the lowest portion of the trunk from November through March will also reduce rodent damage. Plastic guards should be removed in the spring to speed drying of trunks after precipitation and to avoid creating a protected habitat for insects.
Biological control of perennial canker with Alternaria alternata, Trichoderma harzianum, and Epicoccum nigrum has been tried experimentally. However, while these fungi were sometimes effective Leucostoma antagonists, none reduced canker on peach trees. Poultices of brown mustard plants or mustard oil have also been evaluated as organic controls, but oil only protected peach trees for five months, and therefore these options may have limited practicality. When trees have small cankers (less than half the branch or trunk diameter) and dry weather is expected for at least three days, surgical removal may be feasible. During June, when rapid healing occurs, the dead bark tissue can be surgically removed, without penetrating the hardwood. Tissue must be removed at least two inches beyond the lesion, disinfecting blades between cuts. Wounds should not be treated or painted after surgery and diseased wood should be removed from the orchard or burned. Further details of surgical removal of cankers are available at: http://extension.psu.edu/plants/gardening/fphg/stone/diseases/cytospora-canker-of-stone-fruits.
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REVISED: February 15, 2017