Unfortunately, there are thousands of diseases that affect trees. Factors that affect disease include region and season among others. There are different specific diseases found in coniferous trees, deciduous trees, and even shrubs, but there are universal signs of disease in general.
The most common tree pathogens, or organisms that cause disease, are fungi. Fungi are parasites that attack trees because they, unlike trees, cannot produce their own chlorophyll, which is a chemical that helps light to be converted into energy. In deciduous trees, fungi usually manifest themselves as spots on the leaves. The spots can be white, which may suggest powdery mildew, or black which may suggest sooty mold. Fungi can also cause cankers, or an area of bark that is dead and used by the fungi to produce spores. Some fungi even have the ability to decay wood, which causes heart rot. This is a sign of serious decline in the health of the tree.
Fungi also affect coniferous trees. A common killer, needle cast, causes the tree to shed its needles and may eventually kill some of the branches. Before the needles fall off the tree, they may turn purple, yellow, or brown. The fungus, which is white or black depending on the species of the tree, can be spotted developing on the infected needles.
Another common fungal infection in conifers is needle blight. Yellow and/or brown spots on the needles incriminate this fungus. The needles later turn reddish brown to brown and drop off the tree.
In both deciduous and coniferous trees, fungi will attack the roots. Vascular wilt is common, and is caused by the pathogen disrupting the flow of nutrients, such as water and minerals, through the tissue of the tree. The disease interferes with the flow of sap, which causes dehydration throughout the tree, leading to wilting and drought-like effects.
A common non-fungal disease in deciduous trees is leaf scorch. This happens when the roots fail to supply the tree with an adequate amount of water, resulting in brown, dry leaves. This disease is likely to appear during droughts and dry spells, but may also indicate disease or other problems within the roots of the tree. Leaf scorch is similar to the effects of air pollution that may emerge in trees over time, and the two are often mistaken.
Another non-fungal disease that occurs in deciduous trees is slime flux. Ugly and rank smelling, this disease is caused by bacteria. The pathogens produce gas that causes the wood to crack and ooze sap. The infected wood appears to be wet and is discolored.
Insects or mites that feed on a tree can cause irregular plant growths, or galls. Such insects include maggots and wasps. The growths can appear on leaves, buds, roots, flowers, or bark. If they are on the leaves, the galls are usually not detrimental to the tree’s health. When they appear on other parts of the tree, however, they are detrimental as well as difficult to eliminate.