Diseases are categorized here by the tree part they most commonly affect. Strategies for minimizing damage from the more serious diseases are discussed in Chapter 6: Managing Important Forest Types.
Foliage diseases may 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 their 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 in spruce, pine needle rust in conifers, and anthracnose and leaf spot in hardwoods.
When the leaves of a hardwood turn yellow or brown and droop, suspect a wilt disease. These symptoms commonly occur when a fungus blocks a treefs 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. Adjoining trees of the same species will sometimes form grafts between their root systems, allowing not only water and nutrient flow between trees, but also disease transmission.
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. Both damage trees and shrubs by blocking the 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 witchesf 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, though they may reduce wood value for some uses.
Stem and Branch Diseases
Cankers are dead areas on stems that are symptoms of diseases such as nectria canker in maple, hypoxylon canker in aspen, and scleroderris canker in pines. Affected areas may be irregular, sunken, flattened, or swollen. They may crack open and enlarge each year until they completely girdle the stem, killing the tree above the canker.
Rust diseases in pines may cause stem cankers and turn the foliage yellow before killing the tree. White pine blister rust is the most prevalent rust in the Lake States. This stem disease requires gooseberry (currant) as an alternate host.
Dwarf-mistletoe is a parasitic plant that is 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 one 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 methods include destroying all trees in a cutting area and any infected trees within 60 feet, then burning the slash. Figure 7-10. Canker disease causes serious damage to tree stems.
All trees are susceptible to wood rot. The most obvious signs are fruiting bodies (such as conks and mushrooms) that appear after the rot has been active for several years. Decayed wood may either be water-soaked and spongy or dry and crumbly. It usually is discolored. Many decay organisms enter through wounds in a treefs 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.
Root rot causes decline in tree vigor over weeks, months, or years. Twigs and branches die back and leaves appear small and yellowed and may drop or wilt in hot weather. Since the root system is damaged, infected trees do not respond normally to water or fertilizer and are susceptible to windthrow. Root rot also may cause stem decay.