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Stem Canker of Soybean

Stem canker is a serious disease of susceptible varieties of soybeans in Mississippi, especially when environmental conditions favor spore production and infection. Usually these conditions occur in Mississippi when soybeans are planted before May 15. Yield losses of 30 bushels per acre were recorded for very susceptible varieties in MAFES variety trials in 1991. The disease has been very sporadic and losses minor since that year because of increased number of high-yielding varieties with good resistance. However, some stem canker occurs in Mississippi every year, and soybean producers should keep stem canker resistance high on their list of desired varietal characteristics.

Cause

Stem canker is caused by the fungus Diaporthe phaseolorum var. caulivora. The fungus is probably introduced into a field by infected seed. Once introduced, it may be several years before losses occur. Rains encourage development of the fungus on debris from the previous crop, and wind-blown rain disseminates the fungus in the field. Infection may occur on the stems and possibly through the roots.

Even though infection occurs under favorable conditions during the early vegetative growth stages, it is after pod set that foliage symptoms appear. Symptom development depends on several things: variety susceptibility, time of infection, the plant environment after infection, and plant vigor. Early infections generally result in severe losses. Late infections often go unnoticed because plants mature before cankers fully develop. Late infections, although unnoticed, result in an increase of the fungus that causes stem canker.

Symptoms

The first noticeable symptom of yellowing between the veins of leaflets of trifoliate leaves in the upper part of the plant. Other diseases (e.g., phytophthora rot, red crown rot, and sudden death syndrome) produce similar foliage symptoms. However, the occurence of a canker on the main stem is a symptom that distinguishes stem canker from these other diseases.

Examine the main stem closely to find a canker either extending below, above, or up and down from the attachment of a leaf petiole. If you cannot find a canker in association with a leaf petiole on the main stem, examine the main stem from the soil line up for several inches on the plant. The lesion will generally be light to dark brown with a definite margin between the healthy and diseased tissue.

If infection occurs early, diseased plants die. The disease may first appear on a few scattered plants, but its progress in fields of susceptible varieties can be rapid after the first foliage symptoms are noticed. Yield losses result when plants are killed before complete pod fill. Premature plant death results in reduced seed size and number of seed harvested if death occurs before complete pod set.

There are several biotypes of the fungus. Some are capable of causing more severe disease than others. Thus a susceptible variety may react to stem canker more severely in one location than another.

Preventing Losses

Plant a resistant variety if stem canker has occurred in a field in the past. Never plant a very susceptible, or moderately susceptible variety in a field where stem canker is known to have occurred. If you do not know the disease history of a field, it is less risky to plant only resistant or moderately resistant varieties. There are high-yielding resistant varieties available for most situations. Moderately resistant varieties are also available but have more risk of loss than resistant varieties.

If you feel you must plant a moderately susceptible variety, plant during the latter dates suggested for the variety. Although research has shown less damage from stem canker on soybeans planted after June 1, such a delay will not ensure freedom from stem canker. Be aware that a delay in planting may expose you to porr seed bed moisture and other yield-reducing varieties.

Since the stem canker fungus has been found in seed, do not save seed (for planting purposes) from fields where stem canker has been identified. However, search has not been able to show that infected seed will result in significant disease development. It is reasonable to assume, however, that infected seed can introduce the fungus into previously disease-free fields.

No seed treatments are suggested for the control of stem canker, although seed treatment is a suggested practice to promote seed emergence under certain conditions. Consult Extension Information Sheet 1167 Soybean Seedling Disease for conditions for treating seed and for information on available seed treatment fungicides.

Use production practices that reduce the chances of stress conditions during the growing season. Yield reductions from stem canker can be far greater if plants are stressed from drought, cyst nematodes, lack of fertility, or other unfavorable growing conditions.

If no-till soybeans are to be grown in a field where stem canker has occurred, plant a resistant variety. Avoid planting moderately susceptible or susceptible varieties directly in the previous year's crop debris because stem canker is initiated from the fungus that overwinters on plant debris. There is eveidence that cultivation will aid in reducing stem canker. Conventional tillage and clear seedbed preparation are preferred where there is a history of stem canker.

Foliar-applied fungicides are not recommended stem canker control. The suggested timing for using foliar-applied fungicides to control other diseases appears to have little effect on reducing stem canker. There is research-based evidence that certain weeds may explain the occurrence of stem canker in soybeans planted in fields that were previously not in soybeans.

Varieties Resistant to Stem Canker

Soybean varieties differ in their reaction to stem canker and have been classified from resistant to very susceptible based on tests conducted by the Mississippi Agricultural and Forestry Experiment Station. To date, varieties rated as resistant have been stable in their reaction to stem canker. However, varieties rated from moderately resistant to susceptible may vary in their reaction, depending on the virulence (capacity to cause severe disease) of the stem canker fungus present in a field, planting date, and how much climatic conditions favor stem canker development. Lists of varieties and their reactions to stem canker are published annually in the Mississippi State University Extension publication "Soybean Variety Information" Plant Disease Dispatch, Field Crops M-131 or the Mississippi Agricultural and Forestry Experiment Station publication "Soybean Variety Trials." Both publications are available at your County Extension office.

This information was compiled by Joseph A. Fox, Extension Plant Pathologist, and Gabriel Sciumbato, MAFES Plant Pathologist, with the assistance of William F. Moore and Bob Keeling, Consulting Plant Pathologists; Makund V. Patel, Extension Plant Pathologist; Alan Blaine, Extension Agronomist; and Ken Roy, MAFES Plant Pathologist. This information was made possible as a result of partial funding from the Mississippi Soybean Promotion Board.

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Publication 1827
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.
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