Tips on diagnosing field crop diseases (Part I)

Now is the time to read up on crop diseases and tune up your diagnostic skills for 2012.

When plant disease is suspected, look for symptoms and signs. “Symptoms” refers to the way a plant responds to stresses caused by the disease or perhaps by the environment. “Signs” are actual parts of the pathogen visible to the eye. Signs might include ooze caused by bacteria, visible mold, or fruiting structures of fungal pathogens. For example, discoloration of the wheat head byFusarium is a symptom, while visible dark fungal spores caused by loose smut, or Ustilago tritici, are a stage of the fungal pathogen itself and are therefore a sign.

To successfully diagnose plant disease problems, a few simple tools are useful, including:

  • Good hand lens
  • Sharp knife
  • Camera capable of taking acceptable close-up digital photos
  • GPS unit for pinpointing the location of diseased plants or problem areas. Visiting the same location in following years can help manage more persistent disease problems. Your notes should also include a traditional description based on nearby road intersections and convenient landmarks, or something similar.
  • Suitable tool (shovel, spade, etc.) for collecting soil samples and digging up plant roots.
  • A simple, seven-step plan for basic plant disease diagnosis follows.

Step 1: Be sure you know about the crop plant you are inspecting. Not only its basic genus and species, like a corn plant, Zea mays, but any specific variety or hybrid characteristics, like Roundup Ready, leafy gene or BMR (brown midrib) properties.

Step 2: Become familiar with the more common disease problems which can affect your crop in your growing area. There are many publications and internet sites that can help, including the American Phytopathological Society. MSU Diagnostic Services has fact sheets on common Michigan crop diseases.

Step 3: Carefully compare those plants with symptoms or signs to others growing nearby. This will help you describe the differences observed more accurately.

Step 4: Look over your field. Do you see the problem in particular areas over the field? Maybe only in a lower spot, or along a field edge? If so, the problem could be something other than disease, like a drainage or herbicide problem.

  • If it appears that all, or nearly all of the plants in a field are affected, then causes other than disease should be considered carefully.
  • Disease rarely infects all the plants in an area. Soil nutrient, frost, hail or chemical damage could explain it.  Disease infection takes time. If symptoms appear very quickly, be careful to explore other causes.

Step 5: Review the cropping history of the affected area. Could a pathogen have persisted in the field because the same crop was grown here previously? Has the problem occurred in this area before? Or maybe an herbicide carry-over problem could exist.

Step 6: Root rot diseases cause above-ground symptoms and should not be overlooked. Small, yellowing leaves, poor terminal growth and flower or fruit production can be associated with root disease. Affected plants should be dug up carefully and their roots examined. Healthy roots will be white or cream-colored. Diseased roots will appear darker. Comparing root appearance on symptomatic plants to those on nearby, healthy plants may be helpful.

Step 7: The entire plant must be inspected carefully. Note whether the entire plant, or only parts like stems or flowers, have symptoms.

The following sources were used in preparation of this article:

For more information, contact Jim Isleib at 906-387-2530.

Read part two of this two-part article series, Tips on diagnosing field crop diseases (Part II).

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