Identification of Avr Alleles in Leptosphaeria maculans Isolates from Different Oilseed Rape Cultivars

Abstract

This report describes a survey of the regional distribution of races of Leptosphaeria maculans done by characterising isolates from phoma leaf spot samples taken from different oilseed rape cultivars at different sites in the UK. Phoma leaf spot assessment was done on cultivars Drakkar (with no R gene), Bilbao (with Rlm4), Adriana (with Rlm4 and quantitative resistance), Roxet and Excel (with Rlm7) sampled from three sites (Harpenden, Cranwell & Bainton) results showed that cultivar Drakkar had most phoma leaf spots. The cultivar Adriana had fewer phoma leaf spots than Bilbao or Roxet. The identity of alleles at eight Avr loci was determined for L. maculans isolates from these cultivars. Additionally some more isolates from Banbury, Morley and Rothwell in UK and Verpillieres in France were also used for the identification of Avr alleles. The results of the cotyledon tests confirmed that isolates from cultivars Bilbao and Adriana were virulent against Rlm4. An isolate from cultivar Roxet was virulent against Rlm7. The isolates from cultivar Drakkar were virulent against Rlm1, Rlm2, Rlm3, Rlm5 and Rlm9. The frequency of the avirulent AvrLm1 allele was 35% in this study. No isolates of L. maculans possessed the AvrLm2 allele. In this survey it was observed that the frequencies of the AvrLm3 allele (3.8%) and AvrLm9 allele (3.8%) were less than those of the AvrLm4 allele (50%), AvrLm5 allele (84.6%), AvrLm6 allele (96.2%) and AvrLm7 allele (88.5%). This study suggested that Rlm6 and Rlm7 are still effective sources of resistance against L. maculans in oilseed rape in the UK. Seventy-two isolates obtained from nine different oilseed rape cultivars from the sites Harpenden and Spalding in the UK and from Poland were checked for their identity as L. maculans by species-specific PCR and for the presence of the AvrLm1 allele by allele-specific PCR. Sixty-nine of them were confirmed as L. maculans and only four of these sixty-nine isolates (from the cultivar Roxet) had the avirulent AvrLm1 allele.