Environment Planet Earth Common Hardwood Tree Diseases - Prevention and Control Major Classifications of Hardwood Pathogens By Steve Nix Steve Nix Writer University of Georgia Steve Nix is a member of the Society of American Foresters and a former forest resources analyst for the state of Alabama. Learn about our editorial process Updated March 17, 2017 Claire Higgins/Photolibrary/Getty Images Planet Earth Outdoors Weather Conservation Hardwood or deciduous trees can be harmed or killed by disease-causing organisms called pathogens. The most common tree diseases are caused by fungi. Fungi lack chlorophyll and derive nourishment by feeding on (parasitizing) trees. Many fungi are microscopic but some are visible in the form of mushrooms or conks. Also, some tree diseases are caused by bacteria and viruses. Pathogens can infect many different tree species with similar disease symptoms. Powdery Mildew Tree Disease Powdery mildew is a common disease that appears as a white powdery substance on the leaf surface. It attacks all kinds of trees. Trees most commonly affected by powdery mildew are linden, crabapple, catalpa and chokecherry, but almost any tree or shrub can get powdery mildew. Sooty Mold Tree Disease Sooty mold tree disease may occur on any tree but is most commonly seen on boxelder, elm, linden, and maple. The pathogens are dark fungi that grow either on the honeydew excreted by sucking insects or on exuded material coming from leaves of certain trees. Verticillium Wilt Tree Disease A common soil-borne disease called Verticillium alboatrum enters the tree through its roots and causes leaves to wilt. Light colored leaves with a dull appearance are noticeable in early summer. The leaves then begin to drop. The danger is greatest in highly susceptible trees like maple, catalpa, elm and stone fruit. Canker Tree Disease The term "canker" disease is used to describe a killed area in the bark, the branch or the trunk of an infected tree. Dozens of species of fungi cause canker diseases. Leaf Spot Tree Disease Leaf disease called "leafspots" are caused by a variety of fungi and some bacteria on many trees. An especially harmful version of this disease is called anthracnose which attacks many tree species. Heart Rot Tree Disease Heart rot disease in living trees is caused by fungi which have entered the tree through open wounds and exposed bare wood. Usually a conk or mushroom "fruiting" body is the first sign of infection. All deciduous trees can get heart rot. Root and Butt Rot Tree Disease Root and butt rot disease is the most common disease affecting hardwoods. Many fungi are capable of causing root rots and some cause considerable decay of the butts of trees as well. Root rots are more common on older trees or trees which have sustained root or basal injury.