Association of severe Corynespora root rot of soybean with glyphosate-killed ragweed

The soilborne pathogen Corynespora cassiicola was the predominant fungus isolated from severely stunted soybeans adjacent to glyphosate-killed giant ragweed plants (Ambrosia trifida) in Indiana fields. Soybeans adjacent to glyphosate-killed ragweed exhibited dark-brown to black lesions on 90-95% of their roots and hypocotyls. In contrast, soybeans that were not adjacent to dead Ambrosia trifida, or that were adjacent to living ragweed plants, exhibited only 5-10% root rot; and a number of different soilborne fungi in addition to Corynespora were isolated from these roots. Dead ragweed roots generally yielded pure cultures of Rhizoctonia and were not colonized by Corynespora. Koch's postulates were completed in the greenhouse where typical hypocotyl lesions developed in 3-5 days and lateral, "fine feeder roots" were extensively rotted by Corynespora. Soybean yield reduction was related to the density of glyphosate-killed ragweed plants and ranged from 1.5 kg per dead ragweed to 6 kg per dead ragweed in replicated field plots with and without killed ragweed plants. These field observations indicate that glyphosate or metabolites in dying ragweed root exudates modify the soil environment to predispose adjacent glyphosate-resistant soybean roots to severe Corynespora root rot even at temperatures above 20 degree C.