Impact of Brassica napus–Leptosphaeria maculans interaction on the emergence of virulent isolates of L. maculans, causal agent of blackleg disease in canola

Abstract

Canola (Brassica napus) is the major oilseed crop in Canada. Blackleg (Leptosphaeria maculans) causes yield losses annually, with a larger impact in the last two decades caused by breakdown of resistance in growers’ fields. A 4‐year small‐plot field study from 2014 to 2017 in Manitoba investigated the impact of the B. napus–L. maculans interaction on the emergence of virulent isolates towards specific R genes. Doubled haploid B. napus ‘Topas’ introgression lines carrying single R genes were used, with a 2‐year canola–wheat rotation. Blackleg incidence was reduced by 40% in 2017 compared to 2014 for all R genes tested except Rlm4. In 2017, disease severity was reduced between 21% and 52% compared to 2014 for all R genes tested except Rlm2. There was a significant shift from AvrLm2 and AvrLm4 to virulent alleles avrLm2 and avrLm4, respectively, which led to the gain of virulence toward Rlm2 and Rlm4 Topas introgression lines within a year. Sequencing of the AvrLm2 gene of isolates from 2014 and 2015 revealed a shift of AvrLm2 to avrLm2 allele due to accumulation of point mutations. Masking of AvrLm3 phenotype by the presence of the AvrLm4‐7 allele was also confirmed by analysing phenotypes and genotypes of the isolates collected from 2014 to 2017. Based on these and previous findings, we demonstrate the need for an epidemiological model that takes into account complex molecular mechanisms allowing plant breeders to select appropriate R genes, including balanced cultivar rotation strategies to maintain blackleg resistance in the Canadian Prairies.

Validating the Strategic Deployment of Blackleg Resistance Gene Groups in Commercial Canola Fields on the Canadian Prairies

“…These two examples, along with the Canadian Rlm3 breakdown example, show the impact major resistance genes can have on the L. maculans avirulence profile (Zhang et al, 2016). Rashid et al (2020) found a rapid loss of avirulence and shifts to virulence by L. maculans isolates in as little as 1 year in Canada. Identifying the blackleg major resistance genes within a cultivar becomes valuable to help properly steward and increase the longevity of the resistance genes (Van de Wouw and Howlett, 2019).…”

“…Under natural conditions, Kutcher et al (2013) reported disease severity levels of less than 0.5. Trials that are inoculated in Canada can still experience low disease severity levels of less than 1.0 ( Rashid et al, 2020 ). In comparison to provincial blackleg disease survey data, this study had higher disease incidence ( Canadian Plant Disease Survey, 2020 ).…”

“…These two examples, along with the Canadian Rlm3 breakdown example, show the impact major resistance genes can have on the L. maculans avirulence profile ( Zhang et al, 2016 ). Rashid et al (2020) found a rapid loss of avirulence and shifts to virulence by L . maculans isolates in as little as 1 year in Canada.…”

“…Analysis of variance (ANOVA) was used to compare means as an initial statistical analysis tool using the PROC MIXED procedure of SAS 9.4 (SAS Institute Inc., Cary, NC) ( Littell, 2006 ). Disease incidence (DI) was transformed using an arcsine root square transformation, and a log transformation was performed for disease severity (DS) ( Shah and Madden, 2004 ; Ghanbarnia et al, 2011 ; Rashid et al, 2020 ) to improve the normality of data distribution. When ANOVA was significant ( P < 0.05) for DI and DS among resistance gene groups, the means were separated using the Tukey-Kramer test.…”

Blackleg Yield Losses and Interactions with Verticillium Stripe in Canola (Brassica napus) in Canada

“…The deployment of blackleg-resistant cultivars is the most effective strategy for blackleg management [ 8 ]. However, the erosion or loss of resistance has been reported in many canola cultivars, reflecting the emergence of virulent isolates of L. maculans [ 9 , 10 ].…”