Normally, stem canker samples start to appear in the ISU Plant Disease Clinic in late July. This year we may see this disease occur later because of the delayed planting in the spring. 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. However, the disease is still found everywhere in the state each year. In 1994, we received many stem canker samples. Because of the similarity of stem canker symptoms to the symptoms of Phytophthora and other root diseases, stem canker often is misidentified as Phytophthora root rot and sometimes as sudden death syndrome (SDS).
Lesions of stem canker.
Soybean stem canker is caused by a soilborne fungus called Diaporthe phaseolorum. The disease usually appears after late July and continues until plant maturity. A brown, slightly sunken lesion appears at the base of a branch or a leaf petiole on one side of a stem. The lesion expands along the stem and sometimes severely girdles it. Branches or the upper part of the plant can be killed, and the dead plants are most visible after the R3 stage.
Similar to Phytophthora root rot, stem canker within a field is indicated by the presence of dead plants with the leaves still attached. Sometimes, infections start around the soil line resulting in the expansion of lesions from the soil upward. Plants with these types of lesions, therefore, can be misidentified as those infected by Phytophthora root rot. Remember that stem canker does not cause root rot, in contrast to Phytophthora.
Like the fungi that cause SDS, the stem canker pathogen also produces a toxic compound that causes soybean leaves to have interveinal chlorotic spots that are similar to those observed in SDS. In the last two years, we have received a number of samples to be checked for SDS. These samples turned out to be stem canker. The two diseases can be separated by using root and stem symptoms. SDS causes root rot but does not cause stem lesions.
The prevalence of stem canker depends on the amount of rainfall received during the early stages of soybean growth. In the spring, the fungus produces many tiny spores on soybean debris, and the spores are carried by splashing rain to healthy soybeans. In a field, diseased plants are often first noticed in areas where the crop stand is thin. This year, we may see a less than normal occurrence of this disease in most Iowa regions because of the relatively dry weather in late June and early July. Regions where soybeans received abundant rainfall early in the season may have more stem canker than drier regions.
This article originally appeared on pages 142-143 of the IC-476(20) -- July 29, 1996 issue.