An Index of Common Tree Diseases

Cronartium is a genus of rust fungi in the family Cronartiaceae. They are heteroecious rusts with two alternating hosts, typically a pine and a flowering plant, and up to five spore stages. Many of the species are plant diseases of major economic importance, causing significant damage.

This disease attacks pines and is most damaging to plantings of both exotic and native pine species in 30 Eastern and Central States. The fungus is seldom found in natural pine stands. Diplodia pinea kills current-year shoots, major branches, and ultimately entire trees. The effects of this disease are most severe in landscape, windbreak, and park plantings. Symptoms are brown, stunted new shoots with short, brown needles.

An anthracnose fungus, Discula sp., has been identified as the causal agent for dogwood anthracnose. Infection of dogwoods is favored by cool, wet spring and fall weather, but can occur throughout the growing season. Drought and winter injury weaken trees and increase disease severity. Consecutive years of heavy infection have resulted in extensive mortality in both woodland and ornamental dogwoods.

Dothistroma blight is a devastating foliar disease of a wide range of pine species. The causal fungus, Dothistroma pini Hulbary, infects and kills needles. Premature defoliation caused by this fungus has resulted in complete failure of most ponderosa pine plantings in States east of the Great Plains.

Dutch elm disease primarily affects American and European species of elm. DED is a major disease problem throughout the range of elm in the United States. The economic loss resulting from death of high value urban trees is considered by many to be "devastating". Fungus infection results in clogging of vascular tissues, preventing water movement to the crown and causing visual symptoms as the tree wilts and dies. American elm is highly susceptible.

Trees favored by dwarf mistletoe (Arceuthobium sp.) are certain conifers, mainly black spruce and lodgepole pine. Dwarf mistletoe infests significant stands of black spruce in the northern U.S. and lodgepole pine in the Northwest and Rocky Mountains. This mistletoe is the most damaging disease agent in lodgepole pine, causing severe growth loss and increased tree mortality. It is estimated to infest 15 percent of all black spruce stands in the northcentral states.

Elytroderma deformans is a needle disease that often causes witches brooms in ponderosa pine. It is sometimes mistakened for dwarf mistletoe. The disease is restricted to "hard" or "two- and three-needle" pine species. Elytroderma needle cast has also been reported in North America on lodgepole, big-cone, jack, Jeffrey, knobcone, Mexican stone, pinyon, and short-leaf pine.

Fire blight is a serious disease of apple and pear. This disease occasionally damages cotoneaster, crabapple, hawthorne, mountain ash, ornamental pear, firethorn, plum quince and spiraea. Fire blight, caused by the blight bacterium Erwinia amylovora, can affect many parts of a susceptible plant but generally noticed first on damaged leaves.

This disease causes death within five years of a tree's life if a stem infection occurs. Mortality is heaviest on trees less than 10 years old. Millions of dollars are lost annually to timber growers because of the disease. The fungus Cronartium fusiforme requires an alternate host to complete its life cycle. Part of the cycle is spent in the living tissue of pine stems and branches, and the remainder in the green leaves of several species of oak.

Leaf infections called "galls" are bumps or growths caused by the feeding of insects or mites. One particularly common version of this rapid explosion of growth is called the common oak gall and is most noticeable on the leaf, stem and twig of an oak tree. Although these galls may look like a serious problem, most are harmless to the overall health of the tree.