Answers to soybean seedling disease questions

When soybeans start to emerge each spring, questions always arise related to stand reductions caused by seedling diseases. Usually these questions relate to three issues: disease identification, replanting, and herbicide effects.

Identification

Several fungi can cause seedling diseases, either before or after soybean emergence. In Iowa, seedling blight is caused by Pythium, Phytophthora, Rhizoctonia, and Fusarium. The way to determine which fungi cause the problem in a particular field is based on above-ground symptoms.

Damping-off by Pythium.

In terms of symptoms, seedling blight by Pythium is similar to that caused by Phytophthora. You usually cannot separate the two without laboratory tests. When seedling blight occurs, dead seedlings are visible on the ground. Infected plants that die before the true leaf stage will have a rotted appearance (see photo above). If leaves are present, they will be a gray-green color before they turn brown and die a few days later. Diseased plants are pulled easily from the soil because roots are rotted. Soybeans planted in cold, wet soil are most susceptible to infection by Pythium. If a disease occurs in warm conditions, it is more likely caused by Phytophthora.

Rhizoctonia and Fusarium also can cause seedling diseases. Seedling diseases from these fungi are different than those caused by Pythium and Phytophthora. Seedling blight by Rhizoctonia normally appears when warm weather begins to arrive. Unlike Pythium/Phytophthora damping-off, stem discoloration by Rhizoctonia usually is limited to the cortical layer of the main root and the hypocotyl. Infected stems remain firm and dry. Typical symptoms are localized brown- to reddish-brown lesions on the hypocotyl (see photo) and lower stem. The reddish-brown color is a good symptom to aid in diagnosing the disease. Compared with other fungi, Fusarium is a minor problem in Iowa, causing about 10 percent of seedling blight problems each year.

Replanting

Whether to replant depends on the severity of the stand reduction, which can be obtained from agronomists or an ISU Extension publication about replanting soybeans (ask for Pm-1155). If replanting must be done, follow these guidelines to avoid further disease damage.

  • Use a seed treatment to reduce the risk of further damping-off (select chemicals according to the fungi that may be causing the blight).
  • Work the ground to provide a better seed bed, especially if the first planting is in no-till ground.

Sometimes, seed fails to emerge because of fungal attack, which is called seed rot or pre-emergence damping-off. For seed rot, there are no above-ground symptoms to determine which fungus is causing the problem. In this case, use chemicals mixed with several fungicides to target all four fungi mentioned.

Herbicide effects

Improper application of herbicides may result in injury of soybean seedlings and make them more susceptible to fungi attack. When frequent rainfall delays spring field work and planting, I have seen soybeans planted shortly after burndown herbicide application because the growers wanted to plant the beans ahead of the next rainy system. The rain then washed the herbicide down to the seeds, injured the soybeans, and caused severe damping-off. Apply herbicides correctly to avoid this type of seedling disease.

Seedling disease by Rhizoctonia.

Further, damage may extend beyond the seedling stage. It has repeatedly been shown that injury from certain herbicides can lead to greater seedling infection by Fusarium spp. An Indiana study showed an increase of Fusarium root rot of soybean related to stress by certain herbicides. Soilborne fungi such as Pythium, Fusarium, and Rhizoctonia solani are commonly involved in root rot.

This article originally appeared on pages 51-52 of the IC-478 (7) -- May 5, 1997 issue.

Updated 05/04/1997 - 1:00pm