The key to solving a plant problem is proper diagnosis. Many plant injuries look quite similar. Learning as many factors related to a plant's cultural history as you can, such as determining the age of the plant, time since transplanting, any unusual weather conditions or any changes in grade or drainage patterns can help you greatly in accurately diagnosing damage to your plants. All these conditions may influence a plant's health.
We have classified tree and shrub diseases according to the part of the plant they affect.
Leaf Diseases
Leaf diseases generally do not cause permanent damage to their host although they detract from its appearance. Severe leaf injury and defoliation for several successive years may weaken the host and make it more susceptible to attack from other diseases or insects. Leaf spots can be caused by either fungi or bacteria, and may be found on many plants
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Blotch
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Powdery Mildew
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Leaf Spot
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The names of many leaf diseases are descriptive of the type of injury produced. They include: powdery mildew, leaf spot, blight, blotch and rust.
Stem Diseases
Stem diseases may develop cankers, dieback, gall or witches' broom.
Canker diseases produce localized dead areas in the bark of twigs, branches and stems. Cankers may enlarge until they encircle the stem and kill the portion of the stem above the canker. They may develop as discolored areas of the bark, which may become sunken. New cankers may show no visible symptoms on the outer surface. Eventually, bark over cankers will split and show the dead tissue beneath.
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Crown Gall disease
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Gall diseases appear as enlarged growths on tree roots or stems. Fungi or bacteria may cause gall diseases by entering the tree through wounds. Development of galls interferes with sap flow and may cause stunting or death of trees and shrubs.
Dieback refers to the gradual dying of twigs or branches from the tip back. Many factors including galls, cankers, root diseases and unfavorable growing conditions can cause dieback. Treatment for dieback includes removing all dead wood and improving growing conditions.
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Witches broom
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Witches' broom is a disease that produces dense clusters of dwarfed shoots. These broom-like clusters generally grow on swollen or enlarged portions of the branches. Witches' broom occurs on both deciduous and evergreen plants but generally does no harm to the host plant. Fungi or viruses cause witches' broom.
Vascular Diseases
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Oak Wilt
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Vascular diseases are some of the most destructive of all tree diseases. They attack the xylem or phloem of the tree, interfering with the movement of water and food. They can kill a plant in a very short time. Verticillium wilt, Dutch elm disease and oak wilt are vascular diseases. Once a tree has contracted one of these diseases, treatment is necessary to slow down progress of the disease.
Root Diseases
Root diseases weaken or kill the roots of affected plants. Loss of roots deprives the plants of sufficient food and water for normal growth. Branch dieback generally results. There is no cure for root diseases, although improved cultural conditions may stimulate the plant to outgrow the disorder.