The variability in Wisconsin weather during 2007 has led to numerous questions regarding plant diseases. One late-season soybean question that has been popping up regards the occurrence of soybean stem canker. Multiple positive soybean stem canker samples have been received by the Plant Disease and Diagnostic Clinic and based on the location of positive detections, it appears that the majority of soybean stem canker samples formed a “U-shaped” pattern from the northwestern part of the state down to the south, before arcing back northwards towards Outagamie, Brown, and Kewaunee Counties. 

 

The disease: Soybean stem canker is currently divided into two types, northern stem canker, caused by Diaporthe phaseolorum var. caulifora, and southern stem canker, caused by Diaporthe phaseolorum var. meridionalis. Generally, it has been thought that the two forms occur in distinct regions, but southern stem canker has been found in Wisconsin (Li et al. 2004). Losses due to soybean stem canker have been estimated to be from 600,000 to 2.7 million bushels between 2003 and 2005 (Wrather and Koenning 2006). 

 

The symptoms: Soybean stem canker symptoms typically first appear during early reproductive stages as reddish-brown lesions at the base of branches or petioles and are often first noted after the petiole has fallen. As the lesion elongates, it becomes dark brown to black forming a canker that is sunken in appearance. The stem often becomes girdled. Foliar symptoms are due to the production of a toxin by the fungus and include interveinal chlorosis and necrosis. Above and below the site of the canker, tissue remains green (Fig. 1) and while the leaves may wither, they remain attached. Stem canker symptoms around the soil line are often confused for Phytophthora root rot; however, stem canker does not cause a root rot. Also, foliar symptoms due to stem canker may be confused for sudden death syndrome, but the absence of a root rot is indicative of stem canker.      

 

Fig. 1. Soybean stem canker showing late symptom expression of withered leaves that remain attached and green tissue both above and below the infected region.

 

Conditions that favor stem canker development: Environmental conditions during the vegetative growth periods are critical to infection and development of soybean stem canker. Cumulative rainfall events during vegetative growth have been found to be crucial for driving the development of a stem canker epidemic and infection is likely to occur after a rain that is followed by at least 24 hours of dew and temperatures between 70 and 86 ºF (Nyvall 1999).  Plant death increases when dry weather follows infection.

 

Management: The best option for managing soybean stem canker is through the use of resistance cultivars, especially in fields where stem canker has previously been a problem. Research has shown that seed infection due to D. phaseolorum var. caulivora can be as high as 10 to 20%. Seed treatment fungicides may help reduce the introduction of stem canker into a field, but would not control infections in the field. Also, since the fungus overwinters in soybean debris, tillage can reduce the risk of infection. Rotation with corn and small grains, which are not hosts of the fungus, can also reduce the risk of subsequent stem canker infections, but alfalfa is a host and has the potential to enable inoculum to build up. Delaying planting may reduce the risk of infection, but this tactic must be balanced with the potential yield loss due to delayed planting.  

 

References:

 

Li, S., Kurtzweil, C., Grau, C. R., and Hartman, G. L. 2004. Occurrence of soybean stem canker (Diaporthe phaseolorum var. meridionalis) in Wisconsin. Plant Disease 88: 576.

 

Nyvall, R. F. 1999. Field Crop Diseases, Third Edition. Iowa State University Press, Ames, IA.

 

Wrather, J. A., and Koenning, S. R. 2006. Estimates of disease effects on soybean yields in the United States 2003 to 2005. Journal of Nematology 38: 173-180.