Effects of Leptosphaeria maculans race structure and fitness cost of virulence on management of phoma stem canker of oilseed rape

Fitness of virulent (avrLm1 and avrLm4) or avirulent (AvrLm1 and AvrLm4) isolates of Leptosphaeria maculans on Brassica napus without the corresponding resistance genes Rlm1 and Rlm4 was investigated in controlled environment (CE) and field experiments. Results indicate that there is a measurable fitness cost for virulent isolates compared to avirulent isolates in terms of number of lesions, size of lesions, distance grown through leaf tissue towards the petiole in CE experiments and in terms of systemic growth from leaf lesions to stems in field experiments. There were differences between the AvrLm1 and AvrLm4 loci in fitness cost. The optimal temperature for leaf infection was higher for AvrLm4 isolates than for AvrLm1 isolates. There was a cultivar effect on fitness cost of virulence at the AvrLm1 locus but not at the AvrLm4 locus. Results from field experiments suggest that on the same host without the corresponding Rlm1 and Rlm4 genes, AvrLm4 isolates were more fit than AvrLm1 isolates in warmer growing seasons. The fitness cost of virulence at the AvrLm1 locus was generally smaller than that at the AvrLm4 locus, suggesting that the corresponding resistance gene Rlm4 is more durable than Rlm1. Frequencies of avirulent AvrLm1 and AvrLm6 alleles in airborne inoculum for 2006/07, 2007/08 and 2008/09 growing seasons in the UK were investigated using quantitative PCR. There were differences in frequencies of AvrLm1 and AvrLm6 alleles in the three seasons. The detection of changes in frequencies of avirulent alleles provides essential information to guide deployment of cultivars with corresponding resistance genes to minimise the risk of breakdown of novel resistance.