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Diagnosing Plant Damage

  MG Manual Reference
Ch. 5, pp. 6 - 7

Determine that a real problem exists. It is essential that the plant be identified (genus, species and cultivar or variety) so that the normal appearance of that plant can be established either by personal knowledge or by utilizing plant reference books. Many horticultural plants, or structures on those plants such as fruits-seeds, lenticels, etc. may appear to be abnormal to the person who is not familiar with the specific plant. For example, the ‘Sunburst’ honey locust might appear to be suffering from a nutrient deficiency because of its chlorotic yellow-green leaf color, but it was selected because of this genetic characteristic. It is not abnormal for this plant. Therefore, it is not a problem.
Figure 1. Normal vs. Abnormal Needle Drop or Leaf Drop from Evergreens

Figure 1

Nondeciduous plants normally retain their leaves for several years but eventually they fall. This drop is usually gradual and production of new leaves obscures loss of older leaves.

A. Normal - If drop is confined to older leaves, alarm is unnecessary because it is a normal response to a condition of stress (e.g. drought). Unfavorable growing conditions, such as drought, may accelerate leaf fall so that it becomes apparent and of concern.

B. Abnormal - If newly produced leaves are lost, it is a problem. Drop of current year's leaves may result from pathogen or insect attack or from chemical deficiencies or toxicities.
Always compare the typical diseased plant with a healthy or normal plant, since normal plant parts or seasonal changes are sometimes mistakenly assumed to be evidence of disease. Examples are the brown, spore-producing bodies on the lower surface of leaves of ferns. These are the normal propagative organs of ferns. Also in this category are the small, brown, club like tips that develop on arborvitae foliage in early spring. These are the male flowers, not deformed shoots. Small galls on the roots of legumes, such as beans and peas, are most likely nitrogen-fixing nodules essential to normal development and are not symptoms of root-knot nematode infection. The leaves of some plants, such as some rhododendron cultivars, are covered by conspicuous fuzz-like epidermal hairs. This is sometimes thought to be evidence of disease, but it is a normal part of the leaf. Varieties of some plants have variegated foliage that may resemble certain virus diseases. These examples illustrate the importance of knowing what the normal plant looks like before attributing some characteristic to disease. In describing the plant "abnormality", distinguish between symptoms and signs: Symptoms are changes in the growth or appearance of the plant in response to living or nonliving damaging factors. Many damaging factors can produce the same symptoms; symptoms are not definitive. Signs are evidence of the damaging factor (pest or pathogen life stages, secretions; mechanical damage; chemical residues; records of weather extremes or chemical applications; damage patterns). Patterns of damage are excellent signs and are definitive diagnostic clues.
Examine the Entire Plant and Its Community
In defining a plant problem, it is essential to determine the real primary problem. There are foliage symptoms that may occur due to root damage. The primary problem would be root damage, not chlorosis of the foliage-examine the roots. In general, if the entire top of the plant or entire branches are exhibiting abnormal characteristics, examine the plant downward to determine the location of the primary damage. Look for the factor causing the damage at the periphery of the plant damage.
Some pathogens and insects as well as nonliving factors are only damaging if the plant has been predisposed by other primary factors. For example, borers generally only attack trees that are already predisposed by moisture or other physical stress. Premature dropping of leaves by foliage plants (i.e. Ficus benjamina ) and of needles by conifers frequently causes alarm. Evergreen plants normally retain their leaves for 3-6 years and lose the oldest gradually during each growing season (Figure 1). This normal leaf drop is not noticed. However, prolonged drought or other stress factors may cause the tree as a whole to take on a yellow color for a short period and may accelerate leaf loss. If the factors involved are not understood, this often causes alarm. The leaves that drop or turn yellow are actually the oldest leaves on the tree, and their dropping is a protective mechanism which results in reduced water loss from the plant as a whole.

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