Analysing virulent and avirulent Leptosphaeria maculans isolates to understand R gene-mediated resistance in Brassica napus

Abstract

Phoma stem canker (Leptosphaeria maculans) is a major disease on oilseed rape (Brassica napus) in Europe (e.g. it causes losses to UK farmers of > £100M p.a.). Single major gene resistance (R) in B. napus against L. maculans is associated with a gene-for-gene interaction and a number of R genes are deployed in European commercial cultivars to control the disease (Delourme et al., 2006). In commercial practice, R gene-mediated resistance is often not durable since L. maculans populations change from avirulent to virulent. A single L. maculans gene AvrLm4-7 interacts with two B. napus R genes (Parlange et al., 2009). The changes at the AvrLm4-7 locus leading to virulence differ between Rlm4 and Rlm7 (Rouxel & Balesdent, pers. comm.). It has been suggested that Rlm7 is more durable than other R genes commercially available (Clarke, pers. comm.). The fitness cost of virulence against Rlm1 and Rlm4 has been used to investigate durability of these R genes (Huang et al., 2010). This work aims to similarly investigate durability of Rlm7 in the UK. Single pycnidial isolates were obtained from phoma spots on leaves of oilseed rape cultivars carrying the Rlm7 gene (Roxet and Excel) in autumn/winter 2011/2012. Twelve of these UK isolates from Roxet and eight from Excel were characterised at the AvrLm4-7 locus using cotyledon inoculation tests on cultivars carrying Rlm4 or Rlm7 genes. The phenotype was characterised as resistant (0–3.5 on the scale), intermediate (3.6–4.9) or susceptible (5.0–9). Unexpectedly, all isolates obtained from Excel and most isolates from Roxet produced resistant phenotypes on cultivars carrying the Rlm7 gene. Only three isolates from Roxet produced an intermediate phenotype on cultivars carrying Rlm7. On cultivars carrying Rlm4, all isolates from Roxet produced a susceptible phenotype whereas the phenotype produced by isolates from Excel varied. To investigate changes in fitness and aggressiveness associated with changes from avirulent to virulent at the AvrLm4-7 locus, four isolates from Excel and four from Roxet that had produced resistant or intermediate phenotypes were characterised by inoculation onto cotyledons of a susceptible cultivar (Drakkar) and several cultivars carrying the Rlm7 gene (Excel, Roxet and Hearty). Size of lesions and sporulation on the lesions were examined 17 dpi. All isolates produced susceptible phenotypes on Drakkar and resistant phenotype on the three cultivars with Rlm7. No statistically significant differences were observed between different cultivars carrying the Rlm7 gene. Future work will include molecular characterisation of theses isolates and more aspects of their fitness and aggressiveness will be examined. Moreover, more than one susceptible cultivars (e.g. Topas) will be included in the experiments. New UK isolates for testing will be obtained before/after harvest in summer 2012 from phoma stem cankers of cultivars carrying the Rlm7 and Rlm4 genes.