The Crabapple Conundrum: Why Apple Scab Disease Stresses Your Tree, and What to Do About It
Zack Shier – Plant Healthcare Manager
ISA Board Certified Master Arborist OH-6560B
Apple scab foliar disease, caused by the fungus Venturia inaequalis, is a common disease affecting many species of trees. Generally, apple scab causes the most headache on crabapple and apple trees in central Ohio, but can also infect other species(pear, hawthorne, etc).
Trees are infected in the spring as the new leaves emerge from the buds located on the twigs. When new leaves emerge, they are not prepared to defend themselves like matured leaves are. As they expand, they are softer and less robust, which allows the infection of foliar diseases to occur with greater success.
Once infected, round lesions that can be red/orange and turn black or brown with age. Fruits and leaves can both be infected, with infected fruits having a shrunken and/or distorted shape, and being black or brown in color. Typically, infected leaves may also be distorted and have multiple lesions on them, leading to loss of green color, eventually turning yellow and/or brown. Early leaf drop is common on infected trees, starting as early as summer and continuing through the fall period. Severely infected trees can be completely defoliated by late summer, with stressed or unhealthy trees being more common to this issue.
Infected leaves will contain spores of the disease when they drop, and can more easily infect host trees the next year. Raking up and disposing of leaves can help provide some protection against more severe injections. That being said, apple scab fungus is so prevalent in central Ohio that it is unlikely you can avoid infection by simply removing infected leaves.
Treatment for apple scab traditionally involves 2-4 foliar sprays using an appropriately labeled fungicide, in the spring, as new leaves come out. These sprays need to be timed around leaf expansion and weather events, as wet conditions can increase infection rates. While fungicide sprays are still very effective, the overall health of the tree itself is often overlooked. Trees have very good natural defense mechanisms, and while zero infection is rare, improving the overall health of the tree can absolutely improve visual disease symptoms. Modern tree care products are making this easier as well, with specific treatments for things like plant defense and leaf thickness. As always, increasing soil organic matter and having a large mulch ring continuously show benefits to trees when considering disease presence.