Fungi cause most tree diseases, although bacteria and viruses also cause some. Poor air circulation and poor drainage encourage the growth of many fungal diseases. Environmental stresses such as under- or over-watering, drought, poor fertility and extreme cold or heat weaken trees and increase their chances of infection. Trees that have been wounded by lawn mowers, trimmers and even pruners also are more susceptible to infections.
Minor Deciduous Tree Diseases
Numerous fungi and bacteria cause leaf spots that range in color from pale green to dark brown. Leaf spots on deciduous trees usually are not serious, although they can affect the appearance of the infected tree.
Powdery mildew is a common fungal disease that coats infected leaves with a dusty white residue. Although powdery mildew normally is not serious, it can be treated with a fungicide or sulfur compound.
Sooty mold is a fungus that grows on honeydew, a sweet substance secreted by aphids and other sucking insects. Use a pesticide according to the manufacturer's directions to kill the pests, and wash the sooty mold off of the foliage with a strong spray of water.
Major Deciduous Tree Diseases
Fungi that enter deciduous trees through wounds and decay the inner wood cause heart rot. Conks, or mushrooms, growing on a tree indicate extensive disease and decay. Trees with heart rot are weakened and may fall during windy weather. There is no treatment for root rot and infected trees should be removed as a safety precaution.
A soil-borne fungus that enters the roots of trees causes Verticillium wilt. One side or the whole tree may be affected with dull-colored foliage, stunted growth and discolored wood under the bark. The leaves turn light colored or yellow, and may wilt before dropping off. There is no effective control for Verticillium wilt. Infected trees should be removed and the area left unplanted, since the fungus can live in the soil for several years.
Minor Evergreen Tree Diseases
Needlecast is caused by the fungus Rhizosphaera kalkhoffii and infects spruce trees. The needles on the lower branches of infected trees turn purplish-brown to brown and drop off. Although the disease progresses up infected trees and ruins their appearance, needlecast normally is not fatal. Regular applications of a fungicide can help prevent and control needlecast.
Junipers are affected by Kabatina tip blight in late winter to early spring. Infected needles initially turn dull orange, then red or yellow. Phomopsis tip blight occurs in mid- to late spring. Infected needles turn dull red or brown, and then fade to ash gray. Infected tips should be pruned and destroyed to prevent the spread of the diseases. Treatment usually is not needed, although an application of fungicide may help control both types of tip blight.
Major Evergreen Tree Diseases
Cercospora needle blight on junipers causes the needles to turn red or brown on the inner part of the lower branches. Repeated infections can result in complete defoliation and death. Regular applications of a fungicide may help prevent or control Cercospora needle blight.
Dothistroma needle blight is a serious disease of numerous species of pines. Initially infected needles have yellow or tan spots and deep, green bands. The bands turn reddish-brown. Then the whole needle turns brown and drops off. Copper fungicides are effective in the prevention and control of Dothisstroma needle blight.
Sirococcus shoot blight affects several species of evergreens, including pines. The disease attacks new shoots, especially on seedlings and saplings, and causes stem and branch cankers. The base of the needles on infected shoots turns yellow or brown, and the entire needle turns brown and dies. Younger shoots may curl or droop, while branches on older trees may fall off. A fungicide can be applied to small trees to prevent or control Sirococcus shoot blight. Older trees need to be removed and destroyed to prevent the spread of disease.