The weather has finally gotten more seasonal, but the past few months were anything but typical. Our spring was cold, wet, chilly and damp. If we struggled with wearing jackets in early May, how did our trees handle this indecisive weather? While trees enjoy the soil moisture, they also struggle with certain fungi that infect newly emerged leaves during periods of sustained wetness and cool temperatures.
Anthracnose diseases are caused by a group of fungi which thrive in such weather. They can be spread by wind and/or rain splash. The fungus takes nutrients from cells within the leaves, in turn killing the cells and creating a leaf lesion. The lesion expands as the fungus spreads. During periods of sustained leaf wetness and cool temperatures, spores are produced from these lesions which can re-infect the same leaf or neighboring leaves. The result is a tree with leaf lesions so plentiful that it looks like the entire tree may be dying. They can be small dark spots or large yellow, tan, grey, reddish brown or brown blotches. Lesions tend to begin along leaf veins because depressions along veins hold water for a longer period of time prolonging wetness and allowing spores to collect. However, lesions often rapidly expand and several lesions may become conjoined and appear as a single lesion.
A large number of hardwood species such as oaks, maples, ash, walnut, dogwood and sycamore are infected by anthracnose. During a spring like the one we are currently experiencing, sycamore trees seem to fair the worst. In addition, the fungus is able to grow out of sycamore leaves into adjacent twigs where it causes small stem cankers and/or deformed twigs.
For the most part, anthracnose diseases are generally cosmetic and cause no serious long-term damage to the trees. In years of severe disease infection, trees can become unsightly or even appear to be dying. The unsightliness usually bothers homeowners more than it bothers the trees. Some trees respond to infection by prematurely shedding leaves (sycamore and ash), but others retain their leaves until normal leaf drop in the fall (oak). Unless the trees were in poor health before the infection, they should be fine the following spring.
While there is nothing that can be done to suppress the fungus once it has infected the leaves, there is some hope for reducing infections for next year. Anthracnose fungi can stay in fallen leaves laying on the ground over the winter. Raking fallen infected leaves, bagging and disposing of them in another location will reduce the amount of spores available the following spring.