Symptoms. When walking through several of the earlier planted soybean fields at the Northwestern Illinois Agricultural Research and Demonstration Center trifoliate leaflets appeared to be missing significant sections of leaf tissue (Figure). Upon closer inspection, on those same leaflets, seemingly random sections of remaining leaflet tissue had angular lesions that looked reddish brown in the middle and were surrounded by a yellowish 'halo' (Figure). These are symptoms typical of a disease called bacterial blight.

Bacterial blight. Bacterial blight is caused by a bacterium called Pseudomonas sevastonoi pv. glycinea. The pathogen grows best in wet weather when temperatures are between 75 and 79 degrees and stops growing when weather turns hot and dry.  The brownish red lesions are caused by a toxin produced by the bacterium, and although less conspicuous than when on leaves, symptoms can also occur on petioles, stems and pods.

Weather drives disease. This bacterium readily survives in the field on soybean residue. Disease is prevalent although soybean was not grown in 2013 and fields had been worked (disk ripper in fall 2012, chisel plow in fall 2013 and soil finisher in spring each year including 2014).  This may be because of inoculum that remained in soybean residue or because seeds sown this spring may have become infested during harvest activities.

This endemic pathogen lives on plant tissues epiphytically (without causing infection) until temperature and moisture levels are just right to allow the bacterium to enter the leaf tissue through pores used for gas exchange (stomates) or through wounds. Lesions begin as small water-soaked spots and develop into mature lesions if temperatures remain cool. Strong winds favor bacterial blight epidemics by causing bacteria to spread among leaves and plants and by causing wounds.

Yield loss and management. Yield loss caused by bacterial blight has been estimated to be between 4 and 40 percent in the U.S. depending upon variety susceptibility and environmental conditions. Resistance genes have been identified in soybean germplasm. However, perhaps due to the rare occurrence of the very cool, wet weather that favors this disease, few major seed companies provide information about variety susceptibility.  Foliar fungicides WILL NOT help to manage this bacterial disease, however copper-based products, if applied very early on may provide some control.

References: D. Malvick. 1990. Bacterial Foliage Diseases of Soybean. RPD No. 502 University of Illinois

J.B. Sinclair. 1999. Bacterial Blight. In Compendium of Soybean Diseases. 4th Ed. Hartman, G.L., Sinclair, J.B, and Rupe, J.C. eds. APS Press. St. Paul, MN