Tips on diagnosing field crop diseases (Part II)

If you’re stumped diagnosing a crop problem, using the Michigan State University Diagnostic Services lab can help you make a timely and valuable diagnosis.

Let’s look at the four major plant pathogen types:

Fungal pathogens are the most common crop disease problems. Both signs and symptoms may be present. Round leaf spots, stem rots with dry or papery texture, concentric rings on leaves, tissue discoloration and plant wilt can indicate fungal infections. Signs of fungal disease can include small fruiting bodies on affected tissue.

A few field crop diseases involve bacteria. On some plants, bacteria can cause gall formation, irregularly-shaped leaf spots, wilting followed by yellowing and tissue death, or wet rots. Potatoes are vulnerable to bacterial infection, including black leg caused by Erwinia sp. bacteria.

Viruses are usually transmitted by insect or nematode vectors, and are seed borne or transferred by sap when plants are physically damaged. These diseases result in poor performance, but usually don’t kill plants outright. Yellowing or mottling of leaves, stunting or distortion of plant form, or tissue dieback can result.

Nematodes are microscopic roundworms. The vast majority of nematodes does not cause plant disease and are either non-harmful or beneficial to the plant’s soil environment. However, there are a small number of serious plant pathogenic nematodes including stem, root and foliar nematodes. The soybean cyst nematode, Northern root knot nematode, stems and lesion nematodes can affect Michigan crops.

When a plant problem is first noticed and disease may be the cause, it is important not to jump to conclusions. Careful observation of the affected plants, the surrounding plants and the general environment is needed. There are many possible causes for the problem and many questions to answer:

  • Plant nutrient problem?
  • Drainage or compaction issues caused by soil texture?
  • Recent or seasonal weather events?
  • Light quality, such as nearby wood lines?
  • Environmental conditions such as drought, excessive moisture or temperature?
  • Cultural conditions including tillage, planting, cultivation and chemical practices?
  • Animals damaging your crop?

If your efforts don’t bring you to a firm conclusion, consider submitting a plant sample to a good diagnostic lab for evaluation. Michigan State University’s Diagnostic Services lab offers such services. General plant health evaluation will check for the presence of pests, disease and nematodes. More comprehensive nematode testing services are also available. The lab will identify plants such as weeds, evaluate herbicide damage, and test some weeds for herbicide resistance.

Proper sample submission is essential for the lab to make the best diagnosis possible. Herbaceous, or annual, plants should be sent whole, including roots and as much soil around the roots as practical. The roots should be wrapped up in a plastic bag and tied off at the soil line to prevent contact with the rest of the plant. It is best to submit samples early in the week to avoid having your sample waiting, and deteriorating, over a weekend before it can be looked at by lab staff.

Check the Diagnostic Services website for services and fees. As an example, “Plant Health Analysis” basic charge is currently $20. You can also call 517-355-4536.

The following sources were used in preparation of this article:

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

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

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