There are many different causes of tree diseases but most fall within the categories of fungus, bacteria, viruses, or other organisms, including pests. When a healthy tree does become diseased, there are usually many elements that factor in, including soil problems, growth deformities, or weather problems, such as too much or too little moisture or damage from winds and storms.
Identifying tree diseases can sometimes be a difficult task. The same types of diseases often affect both forest trees and backyard trees. But while spraying of a few trees might be deemed feasible, this would be cost prohibitive in forests. Exceptions to this rule are large Christmas tree farms or nurseries, where livelihood is dependent on the trees and there is some hope of controlling the spread of the disease.
Fungi are the number one cause of infectious diseases in trees. Even a healthy tree can succumb to this type of disease. A fungus grows from tiny spores that are too small to see, and these spores are carried from one place to the next by the wind, rain, and even insects, who often carry them from one tree to another.
Rust fungus is a good example. It is aptly rust-colored or reddish orange and travels from one live tree to another. The fungus will die with the tree but chances are, it has already spread to other live trees nearby. Another destructive fungus that causes tree diseases is wood decay fungus. While this fungus can be helpful in ridding the forest of down and dead branches, it can also attack live trees. These fungi grow through damp wood and sometimes even hollow out tree limbs and trunks so that the inside of the tree is destroyed, but it’s not noticed on the outside until the tree is dying.
Canker fungus is another of the most stubborn tree diseases to eliminate. A tree can be infected by a broken branch or a cut that left the inner part of the tree exposed. The fungus then starts to kill the tree bark and it takes for itself vital nutrients that the tree needs to live. Eventually over time, the leaves will start to die and it will produce more and more bare branches each year. This type of tree disease can be caused by a cut from an automobile hitting a tree or a scratch from lawn equipment causing a wound.
Many other tree diseases are caused by different forms of root rot. And, there are all kinds of insects that are intent on destroying trees--worms, caterpillars, beetles, and moths to name a few. Some tree diseases are also introduced into North America by infected plants being imported--perhaps the chief of these was American Chestnut Blight, which was imported from Asia but has destroyed just about all of the American Chestnut Trees.
The top twenty-five most common tree diseases in the United States are: American Chestnut Blight, Amillaria Root Rot, Anthracnose Diseases, Annosus Root Rot, Aspen Canker, Beech Bark Disease, Brown Spot in Longleaf Pine, Canker Rot, Commandra Blister Rot, Cronartium Rusts, Diplodia Blight of Pine, Dogwood Anthracnose, Dothistroma Needle Blight, Dutch Elm Disease, Dwarf Mistloe, Elytroderma Needle Cast, Fusiform Rust, Laminated Root Rot, Littleleaf Disease, Lucidus Root & Butt Rot, Mistletoe, Oak Wilt, Scheroderrus Canker, Sudden Oak Death, and White Pine Blister Rust.