Late-summer soybean diseases

Most regions in Iowa had little rainfall in July and early August, which greatly reduces the occurrence of soybean disease. As the growing season progresses, however, we may find some late-season diseases. Two of them are worth mentioning.

Brown stem rot is a fungal disease caused by the fungus Phialophora gregata. The disease appears late in the growing season. You can identify brown stem rot by splitting the stems longitudinally and checking for internal stem symptoms. Healthy stems have white tissue in the center; infected tissue is brown. Often there may be an internal browning only at the nodes, while internodal tissue may be white. Browning of the tissue progresses upward in the stem during the growing season, so always check the low stem first. Later in the growing season, the lower part of the stem may show external browning with wilting and premature leaf drop. Severely infected stems are rotten and soft with discoloration spreading outward to the stem surface.

Stem with and without brown stem rot.

Some strains of BSR fungus also can cause defoliation in late summer. These strains produce toxins that lead to interveinal necrosis in which tissue between the veins turns brown and dries rapidly, and tissue adjacent to the veins remains green a few days longer. Foliar symptoms of BSR are similar to that of soybean sudden death syndrome.

The BSR fungus is very common and present in at least 95 percent of Iowa soybean fields. Therefore, it is not surprising to find the disease during scouting. The important thing to notice is the severity of the disease. Severe disease causes premature yellowing of leaves, sometimes followed with lodging and defoliation. When the disease is severe, consider practices to manage risk of the disease in the next soybean crop. Management of this disease is relatively easy because of the availability of resistant varieties. A new publication, Soybean Brown Stem Rot, Pm-890, is available from ISU Extension.

Stem canker usually is a mid-summer disease but I have received some samples and reports of this disease since early August. It is caused by the fungus Diaporthe phaseolorum. The lesions are sunken and turn brown on the soybean stem 2-3 inches above the soil. During pod set, leaves just above the canker become yellow. Because the fungus produces a toxin, the leaflets of plants with canker turn bright yellow between veins and turn brown with time. As the infection progresses, the plants become very brittle so that the branches break easily at the infection site. The canker girdles the stem, killing branches and the whole plant.

Stem canker in soybean.

The disease often is scattered in soybean fields. It can be severe where plant density is low because stem canker fungus is dispersed by rain splash; areas with fewer plants have a greater chance of being splashed, resulting in more infections. If the disease is severe, consider changing to a variety less susceptible to the disease. Most cultivars in the north central region are resistant to this disease. If the disease has never been seen on a particular variety, it mostly likely is resistant. The fungus survives primarily as mycelia in infected soybean residue, but once the debris decomposes, the fungus dies in the soil. Fall tillage to promote decay of debris helps reduce the population of this pathogen.

We continue to monitor soybean white mold activity, which was apparently slowed down by weather during July and early August. With the exception of a plot near Mason City in northeast Iowa, the disease appears to be very light at the rest of our research plots throughout the state.

This article originally appeared on pages 166-167 of the IC-478(21) -- September 2, 1997 issue.

Updated 09/01/1997 - 1:00pm