A method for screening soybean seedlings for resistance to northern stem canker caused by Diaporthe phaseolorum var. caulivora

Abstract

Five soybean varieties, Williams, Harosoy, AC Hime, Coles and Leo were inoculated 2 and 4 wk after planting with Diaporthe phaseolorum var. caulivora to determine if differences in resistance to stem canker could be detected in the early stages of plant development. Seedling stems were severed above the unifoliolate leaves and inoculum was applied to the cut surface. Stem lesion lengths were measured at 1, 2 and 3 wk following inoculation. The mean lesion length on seedlings inoculated 2 wk after planting (27.2 mm) was significantly less than seedlings inoculated 4 wk after planting (44.5 mm). Based on mean lesion length, Williams was the most susceptible variety followed by Harosoy. AC Hime, Coles and Leo were equally resistant. Stem canker development measured at 2 and 3 wk following inoculation was negatively correlated with stem moisture at the time of inoculation suggesting that stem moisture is important in the development and severity of stem canker.