Effectiveness of Rlm7 resistance against Leptosphaeria maculans (phoma stem canker) in UK winter oilseed rape cultivars

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.

Interactions in the Brassica napus–Pyrenopeziza brassicae pathosystem and sources of resistance to P. brassicae (light leaf spot)

“…The most resistant cultivar was Imola, followed by the DH lines Q2, Q69, Q83, and cultivars Yudal and Cuillin. Interestingly, cultivars Excel, Hearty, and Harper, with the Rlm7 gene that is very effective against Leptosphaeria maculans (phoma stem canker) (Mitrousia et al, 2018),…”

Effects of cultivar resistance and fungicide application on stem canker of oilseed rape (Brassica napus) and potential interseasonal transmission of Leptosphaeria spp. inoculum

“…These methods could be even more effective if geographical screening work were routinely done to identify L. maculans population structures in different regions for effective deployment of resistance (R) genes to control phoma stem canker. Deployment of cultivars with different R genes will not only reduce phoma stem canker severity before harvest and decrease inoculum concentration for the next cropping season, but can also prolong the lifespan of cultivar resistance (van den Bosch & Gilligan, 2003;Gladders et al, 2006a;Marcroft et al, 2004;Mitrousia et al, 2018). Effective control of severe phoma stem canker epidemics relies on use of fungicides.…”

“…The stems were cut at the crown collar and the severity of stem base canker on each plant was scored using a 0–7 scale (modified from the 1–6 scale of Lô‐Pelzer et al, 2009a), whereby 0 = 0%, 1 = 1– 5%, 2 = 6%–25%, 3 = 26%–50%, 4 = 51%–75%, 5 = 76%–99%, 6 = 100% stem cross‐section affected and plant still alive; 7 = 100% cross‐section affected, dead stem with a hollow or severely necrotic pith. Stem cankers assessed in this work refer to stem basal cankers which were classified as cankers at the root crown or <10 cm above it (Mitrousia et al, 2018). Mean disease severity score per plot was calculated from the 20 plants per plot assessed.…”

Major Plant Diseases Caused by Phoma sensu lato Species and Their Integrated Management Strategies

“…Willd., and Trichoderma harzianum Rifai have been proposed as effective biological control agents of stem canker disease (Aubertot et al 2006). Even though it is still considered a challenging approach, resistant cultivars could be the best long-term strategy to control Phoma stem canker (Mitrousia et al 2018).…”