With thousands of pathogenic fungi capable of infecting plants, fungi are the most common causes of plant diseases, according to the University of Missouri Extension. Trees are affected as well, with diseases ranging from unsightly but mostly harmless mildews to untreatable and fatal wilts and blights. Careful observation and good cultural practices may help you to save a tree from fungal diseases.
Unlike plants, fungi cannot synthesize their own food using energy from the sun. Instead, they must draw their food from a source--sometimes a tree. Fungi extend fibers into the host plant, absorbing its nutrients and depleting its energy, causing the tree to decline. Collectively, these fibers are referred to as mycelia; familiar examples of mycelia include the mold that grows on old food or bread. Visible evidence of mycelia on a tree means the tree might have a fungal disease.
Closely observe your tree and note any abnormalities or symptoms. The Iowa Cooperative Extension Service suggests beginning with an inspection of the leaves or needles. Next, examine the trunk and branches. Signs of decay may show as weak, wet wood or visible fungal growth. Streaky wood may indicate vascular wilt. Mechanical or insect damage provides another way for fungus to invade your tree. Finally, inspect the roots and the soil. Excessive moisture in the soil especially creates optimal conditions for fungus to grow.
Many types of fungal pathogens infect different types of trees. Anthracnose causes leaf spotting, dieback and branch cankers, eventually leading to death on severely defoliated trees. Leaf spot diseases tend to be aesthetic rather than fatal to trees, but many types affect different tree species. Sooty mold and powdery mildew show evidence of fungal covering on leaves and may also indicate insect infestations. Rusts vary in the harm they cause to trees but are unique in that they require multiple host plants to survive. Some fungi cause cankers, sunken wounds on the trunks of trees, or root rot. Vascular wilt fungi invade the vascular systems of trees, impeding the flow of water and nutrients and, eventually, killing the tree.
Whether a tree can be successfully treated depends on the fungal disease afflicting it. According to the Ohio State University Extension, some fungicides do have curative value. Only apply fungicides onto trees for which they are approved and follow all instructions and precautions on the label.
More often, fungicides work best as a preventative measure, applied during times when fungal infection is likely. Good cultural practices also prevent fungal diseases. Keep your trees healthy and do not over water, as wet conditions favor fungal growth. Take care when cutting grass or trimming weeds not to wound the trunks of trees, as these become an entry point for pathogens. Finally, remove dead leaves and branches, which tends to harbor fungus.