Effect of Virulence of Root Rot Pathogens and Cultivar Resistance on Disease Occurrence in Dry Beans

Abstract

Dry bean samples with root rot symptoms were collected in a Z pattern from field locations representing the 11 production counties in west and central Nebraska from 2015 to 2017. Recovered isolates were identified using primers for the β-tubulin gene and internal transcribed spacer region. Among the 132 isolates obtained including, Fusarium (44%), Rhizoctonia (30.3%), and other fungal genera (25.7%), the top 17 pathogenic strains were selected through rolled towel assay and used in a greenhouse study to challenge seven dry bean cultivars in six market classes commonly produced in Nebraska, namely, great northern, pinto, navy, kidney, small red, and black bean. Destructive sampling and disease rating of dry bean plants was conducted to evaluate cultivar resistance, compare virulence among strains, and assess the effect on disease severity. Strains of R. solani AG-5, R. solani AG-2-2 IIIB, R. solani AG-4, F. cuneirostrum, and F. sporotrichioides were the most virulent. None of the tested dry bean cultivars had durable resistance; however, there were significant differences in cultivar susceptibility. The Medalist dry bean cultivar was the least susceptible, and Merlot was the most susceptible. The virulence of the pathogens was more important than the cultivar resistance and should be considered in dry bean production decisions.