Tree diseases take on many different forms and are perpetrated by many different types of organisms. Furthermore, certain species are more likely to develop an infection and within a certain species there may be several varieties or cultivars, which could be more or less disease resistant than the main population. To even further complicate matters, poor growing conditions can initiate a response by the plant, which resembles that caused by a disease pathogen.
Identify the tree. Tree diseases are usually species specific and rarely attack different kinds of species. Once you identify the type of tree, you can usually narrow down the possibilities to a half-dozen pathogens.
Look for shelf fungi growing on the trunk. These fungi are easy to spot and a pretty good sign that a tree has (or will soon have) a serious fungus infection. One course of action is to cut the tree down and harvest the lumber, but often the tree is just left to rot, die and provide home for animals and nutrients for the soil.
Check the leaf to see to see if there is any visual problem. Most likely candidates are insects such as moths, caterpillars, aphids and borers. Knowing the species of tree will help immensely in characterising disease problems. Insects often cause the plant leaf to form galls, which are raised areas and welts. Fungi can also attack the leaf, but this organism may only cause discolouration.
Examine the twigs, buds and small branches for fungus infections such as a rust. A rust is a fungus with a presence like a fuzzy growth, sometimes resembling felt. The rust is often black in colour, but appearance can vary widely. Rust also attacks leaves, causing discolouration or spots.
Go back to the trunk to see if there are any cankers or lesions growing on the bark. Although a fungus in this part of the tree is always a reason for concern, not all discoloured ones on the trunk indicate internal damage to the tree.
Stand back and look at the upper portion of the tree and look for wilt at the top. Treetop wilt can be of two types. One is caused by lack of water and the other by fungal parasite.
Tree diseases are not contagious. A few species are highly susceptible to specific diseases. For example, the American chestnut never reaches maturity because of a blight. Tree diseases are best learnt on a case-by-case basis. Some diseases are benign. For example, the ash tree sometimes succumbs to a bright orange-coloured rust that is not destructive. Defoliation is a natural and often successful reaction to a disease or insect pest. Often the tree will come back healthy the following spring.
Disease identification is not an exact science. Even experts can have difficulty with the process.