Phoma stem canker (Leptosphaeria maculans)

Georgia Mitrousia - The University of Hertfordshire.


Phoma stem canker (Leptosphaeria maculans) is the main disease problem on oilseed rape (Brassica napus) in the UK. The main strategy for control of the disease is effective deployment of a number of single genes for resistance (Rlmx) to the pathogen (Delourme et al., 2006; Aubertot et al., 2006). The disease causes losses to UK farmers of > £100M p.a., despite use of fungicides (Fitt et al., 2006; Stonard et al., 2010). New sources of resistance to L. maculans with R genes are very effective when first introduced into commercial cultivars (Delourme et al. 2006) but are often not durable. There is cost to the breeding industry (resistance breeding costs, loss of new germplasm, c. £5M p.a.) and to HGCA (RL resistance rating, c. £50K p.a.) when resistance breaks down. For a cost-effective use of resistance (R) genes in agriculture, there is a need to understand the durability of R gene-mediated resistance to avoid severe epidemics (e.g.phoma stem canker on oilseed rape).

Recent field experiments suggest that Rlm7 is more durable than other R genes commercially available (e.g. Rlm4, recognised by the same L. maculans Avr gene, Parlange, et al., 2009) and oilseed rape cultivars carrying this gene have now been commercially available for more than seven years (Clarke, 2011). A current BBSRC LINK project (2011-2014) is investigating effects of environment and host background resistance on effectiveness of different R genes under field conditions. Preliminary results show that cultivars carrying the Rlm7 gene had phoma leaf spots in autumn but few or no stem cankers in the following summer. In addition, phoma leaf spots appeared later and were smaller than lesions on other cultivars. However, it is not clear why L. maculans can affect cultivars with Rlm7 at the leaf stage but not reach the stem. This makes Rlm7-AvrLm7 a good system to investigate mechanisms influencing the durability of R gene-mediated resistance.

In the proposed project, effectiveness of Rlm7 resistance will be examined in different genetic backgrounds over three growing seasons in different regions of the UK (BBSRC LINK field experiments), since it has been reported that the durability of R genes to pathogens depends on the genetic background (Brun et al., 2010). Disease severity will be assessed in susceptible (Drakkar) and resistant cultivars (Excel, Roxet with Rlm7 and cultivars carrying another R gene (Rlm4) as reference) and L. maculans isolates will be collected from leaves and stems over three growing seasons. Controlled environment experiments will be done using these pathogen isolates to examine pathogen evolution using an established cotyledon test method (Balesdent et al., 2001) and PCR (from DNA extraction). In addition, the isolates from resistant and susceptible cultivars and from leaves/stems will be used to characterise the pathogen fitness, which is the ability of the pathogen to survive and reproduce. Both factors affect the effectiveness of cultivar R genes and will help to understand the durability of R gene-mediated resistance (Rouxel et al., 2003; Huang et al., 2010).

Successful outcomes of the project will result in an improved understanding of the durability of R genemediated resistance to L. maculans in oilseed rape that will benefit not only farmers by increased yields and avoidance of unnecessary fungicide use, but also breeders by producing cultivars with more durable disease resistance. This project aims to contribute in this by participating in meetings with farmers, breeders and other members of the industry (e.g. HGCA, Co-op Farms, NFU), breeders' open days, workshops and conferences to convey important findings and suggestions.

More info on Georgia's project to follow