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Diagnosing Plant Damage

  MG Manual Reference
Ch. 3, pp. 4 - 5


To determine what factors damaged a plant requires an inquisitive, investigative approach combined with careful observation and the ability to put all the pieces together to reconstruct the event(s) that produced the plant damage. Accurate diagnosis must be made before corrective action can be taken; even if no corrective measures are available, there is satisfaction in simply knowing what the problem is and what its future development might be.
Probability of correct diagnosis based on only one or two clues or symptoms is low. Similarities of symptoms produced on the same plant by completely different factors frequently make the use of symptoms alone inadequate.
In diagnosing plant damage a series of deductive steps can be followed to gather information and clues from the big, general situation down to the specific, individual plant or plant part. Through this systematic, diagnostic process of deduction and elimination, the most probable cause of the plant damage can be determined. Steps to follow in gathering diagnostic information are presented in Table 1. Each step will then be expanded and guidelines presented as we proceed through the diagnostic process. We will first identify the problem, then attempt to distinguish between living and nonliving damaging factors based on the observed damage patterns, development of the patterns with time, and other diagnostic signs. Factors causing plant damage can be grouped into two major categories:
  1. Living factors: living organisms such as pathogens (fungi, bacteria, viruses, nematodes) and pests (insects, mites, mollusks, rodents...). With living factors, "Something is missing, and something is gained."
  2. Nonliving factors: mechanical factors (i.e. breakage, abrasions, etc.); physical, environmental factors (extremes of temperature, light, moisture, oxygen, lightning); and, chemical factors (chemical phytotoxicities, nutritional disorders, etc.).

If we suspect that it is a living damaging factor, we will look for signs and symptoms to distinguish between pathogens and insects. If the accumulated evidence suggests that it is a pathogen, we will seek evidence to distinguish among fungal, bacterial, viral pathogens and nematodes. If the evidence indicates the damaging factor is an insect or other animal, we will seek further evidence to distinguish between sucking and chewing types.
If evidence indicates that the damage is being caused by a nonliving factor, we will seek further evidence as to whether the initial damage is occurring in the root or aerial environment. We will then attempt to determine if the damage results from MECHANICAL FACTORS, from extremes in PHYSICAL FACTORS ( i.e. environmental factors such as extremes of temperature, light, moisture, oxygen), or from CHEMICAL FACTORS (i.e. phytotoxic chemicals or nutritional disorders). Once we have identified the plant and limited the range of probable causes of the damage, we can obtain further information to confirm our diagnosis from reference books, specialists such as plant pathologists, entomologists, horticulturists, and/or laboratory analyses.

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