The three basic causes of disease in trees are fungus, bacteria or virus. According to North Dakota State University plant pathologists R. W. Stack and H. A. Lamey, fungus is the most common of the three pathogens and is the most common cause of tree death. A number of different fungi can cause tree death; some attack a wide variety of trees, while others limit their attack to a certain species.
Fungi begin as microscopic spores. These spores may be spread through the air, dead leaves, rainwater, insects or the soil. Trees that have been wounded are most susceptible to infection. The wound provides an opening that allows either the spores or the insect carrying the spores a way to enter into the tree.
Prevention is the best method of controlling deadly fungus. Trees should be planted in soil that is well-drained and away from swampy areas where fungi often breed. Avoid planting trees in areas where fungus diseases have been a problem, unless you are planting a strain that is known to be resistant to that disease. Rake and thoroughly compost fallen leaves to prevent creating a wintering ground for a deadly fungus.
Various methods of controlling fungus are available. One of the most common is fungicide. Fungicide may be sprayed externally or injected into a tree, depending on the fungus being treated. Make sure any trees or firewood you bring into your garden are disease free. Many fungi are spread from one area to another through infected firewood
Hardwood and Softwood Fungi
Two deadly fungi that can affect both hardwood and softwood trees are Arnillaria root disease and Verticillium wilt. Armillaria root rot symptoms include thinning and discoloring at the crown, resin blisters, white mycelial fans under the bark and honey-colored mushrooms near base of tree in the fall. True firs, Douglas firs and western hemlock are most susceptible to the disease. Verticillium wilt symptoms vary. One of the most common signs of an infection is that the leaves have a dull appearance or turn yellow. Eventually, entire branches may die and, if cut, a ring of discolored wood can be seen.
Deadly Softwood Fungi
Fungi that are particularly deadly to softwoods or conifers include diplodia tip blight, fusiform rust of southern pines, and white pine blister rust. Diplodia or Sphaeropsis tip blight causes short brown needles with black spots on new shoots. Fusiform rust causes tapered, spindle-shaped swells known as galls on branches of the tree. Loblolly and slash pines are most susceptible. White pine blister rust affects pines with five needles per fascicle such as Eastern, Western, sugar and limber pine.
Deadly Hardwood Fungi
Deadly hardwood diseases caused by fungus include anthracnose diseases, Dutch elm disease and oak wilt. Anthracnose diseases typically attack hardwood trees such as dogwood, ash, oak, maple and sycamore. It is usually seen when the weather is cool and wet. Although symptoms will vary depending on the strain, the most common sign is dead areas on a tree. Dutch elm disease spread rapidly and will kill a tree within three years. The first sign of Dutch elm disease is that when the leaves on one or two branches become wilted and yellow. Oak wilt kills thousands of oaks each year in the eastern United States. It is difficult to diagnose since symptoms vary and rarely affect the entire tree.