Northern Stem Canker - Scouting

A diagnostic symptom of stem canker is that green stem tissue is typically present above and below stem cankers. Photo credit: Dean Malvick, University of Minnesota Click on image to view a larger version

Without closer examination, Northern stem canker might be easily misdiagnosed as white mold or other disease based on general field appearance. Click on image to view a larger version Photo credit: Thomas Chase, SDSU

Small black dots (stroma) may appear in Northern stem rot cankers singly or in clusters after plants have senesced. Click on image to view a larger version Photo credit: Thomas Chase, SDSU

 

Early-season

Although infection probably only occurs in the early vegetative stages of soybean development, there are initially no visual symptoms. 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.

 

Late-season

Generally, infections are symptomless until plants reach the reproductive stages of flowering and pod development. At this time canker expansion and plant death may be rapid.

Fields with a notable incidence of Northern 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.

Fields should be checked every two weeks from beginning pod fill to harvest maturity for the presence of stem canker. Northern stem canker may appear at low levels ( < 1% incidence) without being noticed; however, when incidence is higher, symptomatic plants are easily noticed.

 

Symptoms

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 Northern 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.

If you are unsure about the diagnosis, seek assistance from an Extension educator, agronomist, or your state university Plant Diagnostic Clinic..

 

Look-alikes

From a distance, fields with stem canker may be mistaken for other diseases such as white mold or Phytophthora stem and root rot (see Table 1). 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 Northern stem canker.

When individual fields have a high incidence of stem canker (30-50%) symptoms have been mistakenly interpreted as "early maturity".

Table 1. Comparison of symptoms of important late-season soybean diseases: Northern stem canker, brown stem rot (BSR), Phytophthora stem and root rot (PRR), white mold, and Sudden death syndrome (SDS).

Plant Part	Stem Canker	BSR	PRR	White Mold	SDS
Roots	healthy	healthy	root rot	healthy	root rot
Exterior stem	Dark, reddish-brown sunken canker starting at node Green tissue on either side	healthy	Brown lesion that develops from the roots	Lesions, sometimes covered with a white mold - diagnostic	healthy
Interior stem	Slight browning at nodes to completely deteriorated stems Internal tissue may have an orange appearance	Brown pith (center) - diagnostic White tissue below stem surface	Browning of tissue below stem surface	white pith	White, healthy pith Browning of tissue below stem surface
Leaves	General yellowing of leaves No distinct yellow spots or blotches Interveinal yellowiing can lead to dead tissue	No symptoms or yellowing between veins similar to SDS	Some yellowing and wilting	General yellowing leading to dead tissue Leaves remain attached	Yellow spots between veins Increase in size until all tissue between veins is yellow, then brown Leaves remain attached to plant

Adapted from: Agronomy Guide for Field Crops (Soybeans: Brown Stem Rot, Stem Canker and SDS). Ontario Ministry of Food and Agriculture.