U.P. TREE IDENTIFICATION KEY
FOREST HEALTH - TREE DISEASES
Like any biological taxum, trees are prone to a variety of diseases. Some are fatal. Others are more bothersome. Diseases can attack any tree part, but most diseases specialize on particular tree parts, such as leaves, fruits, flowers, stems, and roots. Most diseases are fungal, some are viral or bacterial. Some diseases require alternate (non-tree) hosts to complete their life cycle. Exotic diseases present special problems, as diseases are difficult to contain or eradicate. Chestnut blight and Dutch elm disease are notorious examples that have changed the complexion of the eastern forest. There are far too many tree diseases to provide a complete list on this website. Therefore, only some of the more common or important or severe diseasese will be thumbnailed.
To help identify a particular disease, knowing the correct species of tree is critical. It's also quite helpful to observe the affected tree part, note the symptoms, carefully look at disease fruiting bodies, and realize that diseases have particular life cycles and often change appearance over a period of time. Some diseases will be easy to identify. Others will be more difficult. Sending samples to a laboratory, such as the Diagnostics Lab at MSU, requires a proper sample be taken. Time is important. Secondary disease organisms will begin to act on any tissue, rendering culturing more difficult, especially for diseases which require live tissue.
It's also important to realize that most fungi are either neutral or beneficial to trees. A group of root fungi called mycorrhizae are essential to nutrient uptake of many tree species. Wood rotting fungi are critical in nutrient cycling, as well as providing habitat for many wildlife species. Some forest fungi are edible, and tasty! Lastly, fungal fruiting bodies are often colorful and have interesting forms and life cycles. Artist conks are sought after by crafters. Fairy rings mysteriously appear. Foxfire makes for an eerie night.
Diseases of leaves and needles usually appear as brown patches, curling, cracking, and similar symptoms. Most leave diseases of hardwoods are usually not fatal to trees, especially if they happen in the fall season, after most of the photosynthetic season is done. Season can be important. For example, Anthracnoses tend to occur earlier in the growing season (or near irrigated lawns and golf courses). In the case of softwoods, diagnostic small fruiting bodies can often be seen at the appropriate stage of disease development. Keep in mind that all conifers normally lose their older, weathered needles in the fall. Tamaracks and larches, of course, lose all their needles by late October. Weather is important. Cool damp weather generally favors fungal organisms, especially during early summer. Some insect damage can appear disease-like, such as leafminers, skeletonizers, and certain galls.
Most of these diseases appear as deformities in trunk, stems, and branches. Sometimes odd growth patterns occur. Other times, the tissue may appear sunken and discolored. Heart/wood rots eventually produce a fruiting body in the form of a conk. Any source of stress will leave trees more vulnerable to diseases, including shade, heat, unseasonable cold, soil compaction, insect attack, etc. Insects can be vectors of some diseases, such as Dutch elm disease and oak wilt. Injuries and small wounds invite disease organisms. A wound in a tree never heals. It will remain in the tree until the tree decomposes. Often, the celluose-digesting fungi take-hold and continue to decompose the tree from the inside-out. Different fungi digest different components of the wood. White rots eat the celluose-rich portion of the wood. Red rots digest the more difficult lignins. Of course, these trunk fungi create tree cavities that are used by many wildlife species.
A note about lichens . . . these interesting life forms are often mistaken for fungal fruiting bodies (conks) on the trunks of trees. In fact, they are a symbiotic fungi and alga that we call a lichen. Lichens use the trees simply as substrates, the same as they do for a rock or other surface. Lichens appear crusty or leafy, usually green, and can nearly cover portions of tree trunks.
Much of the biomass of a tree lies underground. Because we cannot easily see roots, we sometimes fail to recognize the importance of root diseases. The soil harbors fungi that will take any opportunity to attack the living root tissues. Drought and other stressors can weaken trees, leaving them more vulnerable to root diseases. Once in a tree, these diseases may take years to kill the tree, or a vigorus tree may be able to repel the invader. On the other hand, most soil fungi are either neutral or beneficial to trees. Mycorrhizal fungi are essential for efficient nutrient uptake by trees. The "humongous fungus" near Crystal Falls, made famous by Johann Bruhn, is a common Armillaria species. Most of a tree's root system lies in the upper 18 inches of the soil and will often extend beyond a distance equal to the height of the tree. Soil compaction from logging equipment, ATVs, lawn tractors, and other sources can stress roots. Trenching and excavation can remove significant portions of a root system.
Bill Cook, Michigan State University Extension
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