Abstract
The Rlm7 gene in Brassica napus is an important source of resistance for control of phoma stem canker on oilseed rape caused by the fungus Leptosphaeria maculans. This study shows the first report of L. maculans isolates virulent against Rlm7 in the UK. Leptosphaeria maculans isolates virulent against Rlm7 represented 3% of the pathogen population when cultivars with the Rlm7 gene represented 5% of the UK oilseed rape area in 2012/13. However, the Rlm7 gene has been widely used since then, representing >15% of the UK oilseed rape area in 2015/16. Winter oilseed rape field experiments included cultivars with the Rlm7 gene, with the Rlm4 gene or without Rlm genes and took place at five sites in the UK over four cropping seasons. An increase in phoma leaf spotting severity on Rlm7 cultivars in successive seasons was observed. Major resistance genes played a role in preventing severe phoma leaf spotting at the beginning of the cropping season and, in addition, quantitative resistance (QR) in the cultivars examined made an important contribution to control of phoma stem canker development at the end of the cropping season. Deployment of the Rlm7 resistance gene against L. maculans in cultivars with QR in combination with sustainable disease management practices will prolong the use of this gene for effective control of phoma stem canker epidemics.
Highlights
Major resistance genes have been widely used to protect crops against fungal plant diseases and many breeding companies have deployed them in their programmes over the last 100 years (Stuthman et al, 2007; Stukenbrock & McDonald, 2008)
The first substantial cultivation of oilseed rape cultivars with the Rlm7 gene in the UK was in 2013, when one cultivar (Cv H; Fig. S2) was grown on 5% of the area
The presence of the Rlm7 gene in other cultivars that each accounted for less than 5% of the surveyed area each year was not examined because these cultivars were not identified in the survey
Summary
Major resistance genes have been widely used to protect crops against fungal plant diseases and many breeding companies have deployed them in their programmes over the last 100 years (Stuthman et al, 2007; Stukenbrock & McDonald, 2008). Widespread use of a single resistance gene often results in adaptation of pathogen populations, which evolve to become virulent, rendering the specific resistance gene ineffective (‘boom and bust’ cycle) (Vanderplank, 1968) This is especially true for pathogens with airborne spores and sexual reproduction that have a high evolutionary potential (McDonald & Linde, 2002). One good example of such a pathogen is Leptosphaeria maculans on oilseed rape (Brassica napus), which has a high evolutionary potential; new strains are produced by mutations at their effector gene (AvrLm) loci, rendering the corresponding major host resistance (Rlm) genes ineffective (Sprague et al, 2006) This pathogen’s evolutionary potential is mainly due to its reproductive system and dispersal ability
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