12:00 pm - 1:00 pm
1-30 Agriculture/Forestry Centre, Agriculture/Forestry Centre, University of Alberta, Edmonton AB
Event details: A graduate exam seminar is a presentation of the student’s final research project for their degree. This is an ALES PhD Final Exam Seminar by Keisha Hollman. This seminar is open to the general public to attend.
PhD with Dr. Stephen Strelkov
Thesis Topic: Assessment of Plasmodiophora brassicae Pathotypes and Virulence on Canola in Western Canada
Abstract:
Clubroot, caused by Plasmodiophora brassicae, is a damaging soilborne disease of canola (oilseed rape; Brassica napus). Canola cultivars with ‘first-generation’ clubroot resistance, derived from the European oilseed rape cultivar ‘Mendel’, were introduced in 2009-10. However, new pathotypes of P. brassicae have since emerged that can overcome this resistance. Pathotyping of field isolates recovered from symptomatic galls using the Canadian Clubroot Differential (CCD) set was conducted on 133 canola crops (126 in Alberta, 6 in Saskatchewan, 1 in Manitoba) in 2019, and 127 crops (113 in Alberta, 13 in Saskatchewan, 1 in Manitoba) in 2020. This work identified 25 unique pathotypes, including seven new ones, six of which are resistance-breaking (3C, 8 G, 8I, 9D, 9E and 9F). While pathotypes 3A, 3H, and 3A continued to predominate, consistent with previous years, other pathotypes, such as 8E, appeared to increase in incidence.
The emergence of new pathotypes capable of overcoming resistance has prompted the development of canola cultivars with alternative resistance traits, collectively known as ‘second-generation’ resistance. To monitor the performance of these cultivars, the virulence of P. brassicae isolates collected from clubbed roots in fields planted with second-generation resistant canola was assessed. Surveys conducted between 2019 and 2021 detected clubroot symptoms in 39 canola crops with second-generation resistance, with average disease severities ranging from 1.6% to 61%. Thirty-six P. brassicae isolates were recovered, each representing a different field. Evaluation of these isolates using the CCD set identified 14 distinct pathotypes. Pathotypes 3A, 9D, and 9E were the most virulent, causing severe symptoms in the majority of hosts tested with second-generation resistance. Further analysis using molecular markers specific to two genetically distinct P. brassicae populations in Canada revealed that 32 isolates belonged to population one, two to population two, and two were admixtures. These findings suggest that the effectiveness of second-generation resistance varies with the cultivar and the virulence of P. brassicae populations, and highlight the need for resistance stewardship, regardless of whether first- or second-generation resistance is employed.
While the CCD set is a valuable tool for identifying of P. brassicae pathotypes, it could be improved by replacing several of the current differentials with genetically homogeneous doubled-haploid (DH) lines. Accordingly, the DH B. napus lines ACS-N39 and LCF-1 were evaluated as potential replacements for two current differentials: the universally susceptible Brassica rapa var. pekinensis cultivar ‘Granaat’ and the resistant B. napus hybrid ‘45H29’. LCF1 clearly differentiated resistant and susceptible responses, with no significant differences in average disease severity across resistance-breaking isolates. Clubroot severity was consistently lower in ACS-N39 compared to ‘Granaat’ for both resistance-breaking and non-resistance-breaking isolates. The results suggest that LCF-1 is a suitable replacement for ‘45H29’, but ACS-N39 is not appropriate as a universal suscept.
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