Geographic variation in Leptosphaeria maculans (phoma stem canker of oilseed rape) populations in the UK

Major resistance (R) gene-mediated resistance against Leptosphaeria maculans is associated with a gene-forgene interaction in which the product of a pathogen effector (Avr) gene is recognised by the product of a host R gene so that the pathogen is unable to infect the host. Therefore R gene-mediated resistance to L. maculans is effective only if the corresponding avirulent allele is predominant in the L. maculans population. To use R genes effectively, there is a need to monitor Avr alleles in L. maculans populations. L. maculans populations were sampled from ten sites in England by single pycnidial isolation from phoma leaf spots on leaves of cultivar Drakkar (no R gene). To determine the Avr alleles in each isolate, a set of differential cultivars/lines with known R genes was used. Eighty-nine isolates have been characterized by cotyledon inoculation. Results show that there were no differences between sites in proportions of avirulent alleles of AvrLm7 and AvrLm6 or virulent alleles of AvrLm2, AvrLm3 and AvrLm9. The populations of L. maculans were 100% avirulent at AvrLm6 and AvrLm7 loci at all sites. This indicates that the corresponding resistance genes Rlm6 and Rlm7 are effective. By contrast, the populations of L. maculans were 100% virulent at AvrLm2, AvrLm3 and AvrLm9 loci. There were differences between sites in frequencies of avirulent alleles of AvrLm1, AvrLm4 and AvrLm5. The frequency of AvrLm1 varied from 0 to 45%, AvrLm4 varied from 20 to 90% between the ten sites. Frequencies of AvrLm5 were 100% in all except for two sites.