No abstract

Light leaf spot (Pyrenopeziza brassicae)is an important disease on winter oilseed rape crops (Brassica napus) in northern Europe. In regions where economically damaging epidemics occur, resistance to P. brassicae in commercial cultivars is generally insufficient to control the disease without the use of fungicides. Two major genes for resistance have been identified in seedling experiments, which may operate by decreasing colonisation of B. napus leaf tissues and P. brassicae sporulation. Much of the resistance present in current commercial cultivars is thought to be minor gene-mediated and, in crops, disease escape and tolerance also operate. The subtle strategy of the pathogen means that early colonisation of host tissues is asymptomatic, so a range of techniques and molecular tools is required to investigate mechanisms of resistance. Whilst resistance of new cultivars needs to be assessed in field experiments where they are exposed to populations of P. brassicae under natural conditions, such experiments provide little insight into components of resistance. Genetic components are best assessed in controlled environment experiments with single spore (genetically fixed) P. brassicae isolates. Data for cultivars used in the UK Recommended List trials over several seasons demonstrate how the efficacy of cultivar resistance can be reduced when they are deployed on a widespread scale. There is a need to improve understanding of the components of resistance to P. brassicae to guide the development of breeding and deployment strategies for sustainable management of resistance to P. brassicae in Europe.

The phenotype of the R gene-mediated resistance derived from oilseed rape (Brassica napus) cv. Imola against the light leaf spot plant pathogen, Pyrenopeziza brassicae, was characterized. Using a doubled haploid B. napus mapping population that segregated for resistance against P. brassicae, development of visual symptoms was characterized and symptomless growth was followed using quantitative PCR and scanning electron microscopy on leaves of resistant ⁄ susceptible lines inoculated with suspensions of P. brassicae conidia. Initially, in controlled-environment experiments, growth of P. brassicae was unaffected; then from 8 days post-inoculation (dpi) some epidermal cells collapsed ('black flecking') in green living tissue of cv. Imola and from 13 to 36 dpi there was no increase in the amount of P. brassicae DNA and no asexual sporulation (acervuli ⁄ pustules). By contrast, during this period there was a 300-fold increase in P. brassicae DNA and extensive asexual sporulation in leaves of the susceptible cv. Apex. However, when leaf tissue senesced, the amount of P. brassicae DNA increased rapidly in the resistant but not in the susceptible cultivar and sexual sporulation (apothecia) was abundant on senescent tissues of both. These results were consistent with observations from both controlled condition and field experiments with lines from the mapping population that segregated for this resistance. Analysis of results of both controlled-environment and field experiments suggested that the resistance was mediated by a single R gene located on chromosome A1.

Eight winter oilseed rape and two spring oilseed rape field experiments were performed in the UK in harvest years 2009-12. Each experiment consisted of at least one hybrid and one open-pollinated variety grown at five seed rates from 10 or 20 seeds/m 2 to 160 or 200 seeds/m 2 . Linear plus exponential curves were used to describe the yield response to seed rate and to calculate economically optimal seed rates. Plant counts were then used to derive optimal plant population densities. These ranged from <10 to 39 plants/m 2 for six winter oilseed rape experiments between 73 and >155 plants/m 2 in two winter oilseed rape experiments with severe spring droughts, and from 47 to 65 plants/m 2 for spring oilseed rape. Optimal plant population densities were lower for hybrid than for open-pollinated varieties, due to a combination of the higher cost of hybrid seed and, for some experimental sites, hybrid varieties compensating better for low plant populations. Across all sites, sowing winter oilseed rape at 30 seeds/m 2 rather than common commercial rates of 70 seeds/m 2 for hybrids and 100 seeds/ m 2 for open-pollinated varieties would have increased average gross margin by £29/ha. Sowing spring oilseed rape at 70 seeds/m 2 rather than commonly used rates of 120 or 150 seeds/m 2 would have increased average gross margin by £64/ha.