Many of the soybean samples that have been submitted to the ISU Plant Disease Clinic show signs of hail injury, due to the severe storms and associated hail damage earlier this summer. Disease scouting in late July and August may reveal more stem diseases than usual in fields that had hail injury because wounds from the hail are ideal entryways for some pathogens. Stem canker is one stem disease to be concerned with after such weather conditions.

Lesions of stem canker in soybean.

Soybean stem canker is caused by a soilborne fungus called Diaporthe phaseolorum. The prevalence of stem canker depends on the amount of rainfall received during the early stages of soybean growth. The fungus produces many tiny spores on soybean debris, and the spores are carried by splashing rain to healthy soybeans. Infected plants will not have symptoms until midsummer, often late July. Observations indicate that hail injury may enhance the development of this disease. This year, we may see a higher-than-usual occurrence of soybean stem canker in some fields because of the abundant rainfall and severe storms early in the season.

Plants with stem canker are often first noticed in field areas where the crop stand is thin. An infected plant will have one brown, slightly sunken lesion at the base of a branch or a leaf petiole on one side of the stem. The lesion expands along the stem and sometimes severely girdles it. Branches on the upper part of the plant can be killed, and the dead plants are most visible after the R3 stage.

Phytophthora root rot.

There are similarities between symptoms of stem canker and Phytophthora stem rot. Stem lesions of both diseases are brown to chocolate brown. Both diseases are indicated by the presence of dead plants with the leaves still attached. Leaf symptoms of the two diseases are characterized by chlorotic spots and yellowing. Because of these similarities, stem canker often can be misidentified as Phytophthora root rot. When this misidentification happens in a field planted with a Phytophthora-resistant cultivar, the cultivar would be wrongfully considered no longer resistant to Phytophthora in that field.

Phytophthora-like stem canker.

This year we may have some calls on determination of Phytophthora and stem canker because of the favorable weather conditions for these two diseases. The storms and periodic rains already have resulted in Phytophthora damping-off, which is followed by the stem rot phase of the disease. Phytophthora is prevalent in some areas.

Stem symptoms of stem canker are different from those of Phytophthora in the position of the lesions. Lesions of stem canker start from nodes whereas lesions of Phytophthora stem rot grow from the soil line up because infections occur on the roots. Occasionally, infections by the stem canker fungus start around the soil line resulting in the expansion of lesions from the soil upward; thus, they look like Phytophthora stem rot. Plants with these Phytophthora-like lesions, therefore, are more likely to be misidentified as Phytophthora root rot. In this case, we should pull out infected plants to examine the roots. Stem canker does not cause root rot, in contrast to Phytophthora.

Stem canker was a major problem in the north central region in the 1950s. The use of resistant material has reduced its significance in northern soybean production regions. Although there are observations indicating an increase of this disease in no-till fields, the disease is not a major threat to Iowa soybean production in general. If severe stem canker damage does occur in individual fields, consider managing infested residues by tillage or rotation. The risk of stem canker in the next soybean crop usually is low in tilled or rotation fields.

Differences between stem canker and Phytophthora root rot.