Improving Verticillium longisporum inoculation protocols and quantifying canola yield losses

Abstract

Verticillium stripe, caused by Verticillium longisporum, is an emerging soilborne disease of the Canadian canola (Brassica napus) crop. This study aimed to refine techniques for inoculating the pathogen on canola and to quantify its impact on hybrid canola yield under greenhouse and field conditions in western Canada. Two inoculation methods, root-dip and grain inoculation, were compared in greenhouse experiments with three canola genotypes. Symptoms of Verticillium stripe appeared at early growth stages following root-dip inoculation, resulting in seedling mortalities ranging from 13% to 24% at 35 days post-inoculation. Mortality was significantly greater at higher inoculum concentrations using the root-dip method. Host age at inoculation did not significantly affect disease development in most cases. The grain inoculation method did not cause early-stage mortality, but disease severity at the adult stage differed significantly between control and high inoculum treatments for all host genotypes, with decreases in plant dry weight and height corresponding to increasing inoculum concentration. Grain inoculation may be preferable for large-scale resistance screening or field studies. The relationship between Verticillium stripe severity and yield was investigated in two canola hybrids at two infested field sites near St. Albert, Alberta, in 2020 and 2021. Moderate levels of disease were observed in both hybrids in 2020, while symptoms were milder in 2021. Regression analysis indicated a decline in seed yield per plant with increasing Verticillium stripe severity in both years, best described by second-degree quadratic equations for both hybrids. At one site in 2020, yield loss per plant exceeded 60% in both hybrids when Verticillium stripe severity was > 3 on a 0-4 scale. Similarly, yield loss per plant often exceeded 50% in both hybrids at both sites in 2021 when disease severity was ≥ 1. These findings suggest that V. longisporum can cause significant yield losses even with relatively mild disease severity.