Although infection probably only occurs in the early vegetative stages of soybean development, infections are generally symptomless until plants reach the reproductive stages of flowering and pod development. Some researchers report small reddish lesions on cotyledons. Early infection may spread into the stem, causing seedlings to wither and die. Seedlings may also die before emergence.
Fields with a notable incidence of stem canker may be detected at any time from flowering to well into pod set and development. This usually is sometime from late July onward. In South Dakota, stem canker fields are typically reported in mid- to late August. At this time canker expansion and plant death may be rapid.
Fields should be checked every two weeks from beginning pod fill to harvest maturity for the presence of stem canker. Stem canker may appear at low levels ( ‹ 1% incidence) without being noticed; however, when incidence is higher, symptomatic plants are easily noticed.
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.
Scout for stem canker in the late season by looking for plants on which blacked leaves remain. These plants stand out against the background of healthy plants that have matured and undergone leaf drop. Inspect these “black leaf” plants for lesions and fruiting bodies (stroma) of the fungus at the lower nodes.
From a distance, fields with stem canker may be mistaken for other diseases such as white mold or Phytophthora stem and root rot. For example, lesions can be found at the soil line, making it possible to confuse this disease with Phytophthora stem and root rot. In this case, pull out infected plants to examine the roots. The stem canker pathogen does not cause root rot, while Phytophthora does. Individual plants must be examined closely in order to confirm stem canker.
For a complete discussion and comparison of symptoms of important late-season soybean diseases (stem canker, brown stem rot (BSR), Phytophthora stem and root rot (PRR), white mold, and Sudden death syndrome) please see Stem Canker, CPN 1006, 2015.
If you are unsure about the diagnosis, seek assistance from an Extension educator, agronomist, or your state university Plant Diagnostic Clinic.