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Common Names: northern stem canker and southern stem canker

Causal Agent: Diaporthe phaseolorum pathovar caulivora causes northern stem canker, and D. phaseolorum var. meridionalis causes southern stem canker. These pathogens are in the phylum Ascomycota and produce ascospores.

Host Range: Soybeans and cotton are the most common commercial host for the pathogen, but the pathogen is also known to infect weeds, including morning glory, prickly sida, and black nightshade. Hairy indigo and hemp sesbania possess cankers similar to those found on soybeans when infected.

Geographical Range: Stem canker is found in North and South America, and Europe.

Symptoms:

The first symptoms found on a plant are often reddish lesions on the stem, usually near a leaf node. As the canker ages, the center becomes grayish-brown, with a reddish margin. In northern stem canker, the canker girdles the stem, causing the plant to die above the point of the girdling. The leaves stay on the plant after death. Southern stem canker usually results in the canker elongating on, but not girdling, the stem. Interveinal chlorosis occurs in the leaves as the canker elongates. A phytotoxin causes the chlorosis, and may be the cause of plant death. 

Dissemination:

Diaporthe phaseolorum can be disseminated by the use of infected seed, from debris left to overwinter in the field, or by fallen leaves during the growing season. The major form of dissemination is from the ascospores produced on the debris on the soil surface. The secondary inoculum produced in summer is not believed to affect progression of the disease.

Disease Cycle:

Diaporthe phaseolorum can cause severe damage to a crop by infecting it early in the season, most likely from the spores present in the debris of past crops and weeds. The plant shows no symptoms after inoculation in the spring. Warm and wet conditions are ideal for this pathogen to infect and grow. The disease then progresses by girdling the stem or elongating on the stem, and producing phytotoxins. The pathogen reproduces on the dead and dying material of the host, and can also infect the seeds of the host and cause disease in next year’s crop.

Control Measures:

The best way to control the stem canker disease is by planting resistant cultivars. Resistant cultivars have nearly eliminated the northern stem canker, but the southern stem canker continues to be a problem. Four resistance genes have been found in soybeans for Diaporthe phaseolorum. All of the genes are dominant and are independent of each other, which makes it somewhat easy to cross them into current soybean varieties. Delayed planting and deep plowing are also effective ways to prevent infection in fields with inoculum in the debris.

References:

• Black, B.D.; Padgett, G.B.; Russin, J.S.; Griffen, J.L.; Snow, J.P.; and Berggren, G.T., Jr. 1996. "Potential Weed Hosts for Diaporthe phaseolorum var. caulivora, Causal Agent for Soybean Stem Canker." Plant Disease. 80:763-765.
• Hartman, G.L.; Sinclair, J.B.; and Rupe, J.C. 1999. "Stem Canker."  Compendium of Soybean Diseases, Fourth Edition. 33-34.
• Tyler, J.M. 1995. "Additional Sources of Stem Canker Resistance in Soybean Plant Introductions." Crop Science. 35:376-377.

This paper is an excellent source of information on the advances in disease resistance in plant breeding programs. The study involved crossing susceptible varieties with resistant varieties that possessed the genes Rdc 1, Rdc 2, Rdc 3, and Rdc 4, which are believed to be responsible for most of the resistance to Diaporthe phaseolorum var. caulivora in current soybean varieties. Test crossing the resistant varieties to those of the susceptible plants resulted in a 3:1 ratio of resistant to susceptible plants for each of the four resistance genes. Crossing these offspring with each other resulted in a 15:1 ratio, which demonstrates that the genes are from different alleles and are independent.

Tyler gave insight into the development of resistant plants, and proposed important ideas that could simplify the process of incorporating stem canker resistance into soybeans.