Diseases are categorized here by the tree part they most commonly affect.
Foliage diseases my cause conifer needles to turn yellow or brown or drop prematurely. Hardwood leaves may develop yellow, brown, or black spots. These diseases weaken trees by reducing the ability of leaves to produce plant food. Brown spot disease affects only red and Scotch pines and is typically confined to the lower half of the tree. Some other typical foliage diseases are rhizosphera needle-cast of spruce, pine needle rust on conifers, and anthracnose and leaf spot on hardwoods.
When the leaves of a hardwood turn yellow (or brown) and droop, a wilt disease may be present. These symptoms commonly occur when a fungus blocks a tree’s water-carrying vessels. Oak wilt and Dutch elm disease, verticillium, dothiorella, and phloem necrosis are typical wilt diseases. Oak wilt and Dutch elm disease are serious problems in woodlands and often spread to adjacent trees through root grafts.
Sooty mold is a black powdery fungus that lives on the honeydew exuded by aphids or scale insects. Powdery mildew is a white fungus that covers leaf surfaces. They damage trees and shrubs by blocking sunlight needed for photosynthesis. Their damage is a minor problem in woodlands, but may be serious on some ornamental trees and shrubs.
Abnormal growth, including leaf curling; gall formation on leaves, twigs and fruits; and witches’ brooms (excessively dense branch and twig growth) are the result of high concentrations of plant growth-regulating compounds caused by insects, herbicides, or disease organisms. These conditions are rarely serious by themselves.
Stem and Branch Diseases
Cankers are dead areas on stems that are symptoms of diseases such as nectria canker on maple, hypoxylon canker on aspen, and scleroderris canker on pines. Affected areas may be irregular, sunken, flattened, or swollen. They maycrack open and enlarge each year until they completely girdle the stem, killing the tree above the canker.
Rust diseases on pines may cause stem cankers and turn the foliage yellow before killing the tree. White pine blister rust is the most important rust in the Midwest. This stem disease requires gooseberry (currant) as an alternate host.
Dwarf-mistletoe is a parasitic plant causing a problem in the lake States. It grows on limbs and small branches and may stunt, deform, or kill conifers. Its visible growth is less than 1 inch long and may be either single-stemmed or branched and yellow, brown, or olive green in color. Black spruce stands are the most common host for dwarf mistletoe in the lake States. Control includes destroying all trees in a cutting area as well as any infected trees within 60 feet and burning slash.
All trees are susceptible to wood rot. The most obvious signs are fruiting bodies (conks, mushrooms, etc.) that appear after the rot has been active for several years. Decayed wood may appear water-soaked and spongy or dry and crumbly. It usually is discolored. Many decay organisms enter through wounds in the stem or roots. These rots do not kill trees, but they can destroy the commercial value of the wood. Tree stems with rot are more easily broken by the wind and so can be hazardous.