Michelle Grabowski, University of Minnesota Extension Educator
Across Minnesota, crab apple trees are nearly leafless due to high levels of apple scab this year. Apple scab is a common fungal disease caused by Venturia inaequalis. In the landscape, apple scab can infect flowering crab apple (Malus sp.), hawthorn (Crataegus sp.) and mountain ash (Sorbus sp.) trees.
Apple scab infects leaves and fruit of susceptible trees. Leaf infections are olive green to black spots with feathered edges. Severely infected leaves or leaves with an infected petiole, yellow and fall off prematurely. This early defoliation not only reduces the ornamental value of the tree in the landscape but weakens the tree. Severely defoliated trees are likely to have fewer blooms the following year. Repeated years of defoliation can predispose a tree to winter damage.
Although many Minnesotans don't notice apple scab until July when leaves start to fall from the tree, the disease actually starts early spring just as the buds are opening. In fact the first infections occur before the leaves have completely spread out. New infections peak in the time period between when pink flower buds are visible on the tree and when the petals finally fall from the blossoms. The apple scab fungi need warm wet weather to start new infections and this year provided ideal conditions for disease.
Although we cannot predict what the weather will be next spring, we do know that there will be an abundance of apple scab spores to start the disease all over again. The apple scab fungi survive from one season to the next in infected leaves that have fallen from the tree. The high level of disease this year means that there are many infected leaves going into the winter. Gardeners can reduce the amount of fungi surviving to next year with several simple strategies. All of the leaves should be raked up and removed from below the tree. These leaves can be composted, buried, or burned depending on city regulations. Alternatively the leaves can be mulched with a mulching lawn mower to speed up breakdown. Applying a nitrogen fertilizer to the leaf litter has been shown to further speed up leaf break down and reduce disease the following year.