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Diagnosing Plant Problems

by Phil Pratt, OSU County Extension Office

Diagnosing plant problems can be set forth in some simple steps. However, one of the things that must be kept in mind is that "plant problems" can be caused by several different factors. The first steps in diagnosing plant problems is to remember plants can be made to look or grow abnormally by things that may be separated into three broad categories:

  1. Adverse environmental conditions.
  2. Insect infestation.
  3. Disease infections.

A basic and important step when inspection a "sick" plant is to determine if the cause is environmental or non-environmental (insect/disease) in nature. One key that can help answer this question is looking for "uniformity". Problems caused by adverse environment or unhealthy growing conditions often appear "uniform". Examples of uniformity are...

  1. Many different species of plants in the local area are affected.
  2. Symptoms isolated to specific parts of affected plants (ex. older leaves or leaves isolated on one side of the plant).
  3. All affected plants are exhibiting the same or similar symptom(s).
  4. Symptoms appeared within similar or uniformly time frame.

Uniformity often eliminates insects or pathogenic diseases. It often indicates an adverse environmental factor such as...

  1. Drought.
  2. Improper irrigation (too little or too much water).
  3. Improper fertilization.
  4. Harmful air and/or soil temperature.
  5. Damage from improper use of pesticides or other chemicals.

Insects usually do not feed uniformly on plants and disease causing organisms usually do not cause symptoms that appear uniformly on plants.

Once environmental factors have been ruled out, the next step is to determine if the damage is being caused by insects or diseases. In many cases, if insects are involved they can be seen on the affected plant. But this is not true in every instance. Some insect damaged can be caused by insects that are small and difficult to see, or they may feed at night and drop to the ground during the heat of the day. Close inspection is often required to determine if the cause of the problem is or is not insects.

When you feel comfortably certain insects are not the culprit and adverse environment problems have been eliminated, you are left with pathogen diseases. In general pathogenic diseases are not uniform in relation to when and where they infect a plant. For example a fungal leaf spot disease is often host specific, it will often not infect every leaf on an individual , the infection sites occur randomly on each infected leaf. If the example is a root rot, they usually do not infect every plant within a given population.

In conclusion, when trying to diagnosis plant disease problems, a process of elimination can ofter be used. Check for and eliminate environment related problems and onsect pests. Determine if the problem is uniform in how it is affecting plants. Obvious uniformity lends itself to problems caused by something other than a pathogenic disease. If there does not appear to be uniformity, than consider a pathogenic disease and remember most pathogenic plant diseases are the result of fungi.