Understanding Pyrenopeziza brassicae Pathogen Populations to Improve Control of Light Leaf Spot in Oilseed Rape (Brassica napus)

This project aimed to improve understanding of host resistance against the light leaf spot pathogen Pyrenopeziza brassicae within oilseed rape (Brassica napus) by investigating factors contributing to disease development, studying specific host-pathogen interactions and by identifying quantitative trait loci (QTL) related to host genetic resistance. Patterns of P. brassicae air-borne ascospore release in the whole year over three cropping seasons suggested two major periods of ascospore release: one in late autumn and a second one in early summer. Conversely, spores were absent in winter, but a small number of ascospores that were released in spring may be due to a sufficient number of apothecia produced in early spring. Optimal weather conditions for ascospore release were temperature (ranging between 15-17°C) and associated with rainfall. Next, controlled environment experiments using plants aged from one- to six-weeks-old suggested that four-weeks-old (with four to five true leaves) was the optimal age for disease development and a P. brassicae inoculum concentration of 105 spores/ml was optimal for inoculation without affecting comparable disease assessment between cultivars. Inconclusive results suggested that P. brassicae infection may affect host plant height in controlled environment, as plants inoculated with a greater P. brassicae inoculum concentration (105 spores/ml) were shorter than those inoculated with a smaller inoculum concentration (104 spores/ml). However, the findings could not be reproduced, suggesting that environmental conditions may have influenced the plant growth. Finally, greater light intensity promoted the development of light leaf spot symptoms in controlled environments compared to lower light intensity, suggesting light intensity is an important factor affecting the rate and severity of light leaf spot development. Results from field experiments suggested that there are regional differences in P. brassicae pathogen races. Cultivars Poh Bolko, Castille and Yudal showed greater disease severity in Norwich, Norfolk than in Harpenden, Hertfordshire, suggesting P. brassicae pathogen races in Norwich were more virulent than those in Harpenden. However, cultivars Campus, Catana, Kielder and Acacia remained symptomless across all sites tested, suggesting stable resistances. Similarly, disease severity was greater in Hereford, Herefordshire than in Huntingdon, Cambridgeshire for both moderately resistant cultivars Aquila and Flamingo, possibly due to increased rainfall in Hereford, compared to Huntingdon. Next, inoculation of single-spore P. brassicae isolates in glasshouse experiments suggested stable disease resistance in cultivar Imola, breeding line Q02 and line NPZ 06/22, while cultivars Charger, Bristol, Yudal and Barbados were susceptible to most of the isolates tested. Additionally, 24 single-spore P. brassicae isolates from six European countries, including the UK, were tested and showed differences in virulence between those countries; however, only few isolates per country were tested, so further work is needed to verify these findings. Next, P. brassicae was able to cross-infect between oilseed rape and kale, but oilseed rape-derived P. brassicae inoculum was more virulent than kale-derived P. brassicae inoculum. Finally, results showed that P. brassicae was able to continuously re-sporulate up to four times after removing the previous asexual sporulation until the leaf tissue fully senesced; investigating the inoculum produced by each re-sporulation cycle could provide further insight into the impact of light leaf spot secondary infections. Segregation of resistance against P. brassicae was found within the DY (Darmor-bzh x Yudal) doubled haploid (DH) mapping population in two glasshouse experiments using two different P. brassicae populations (one from England and one from Scotland). Composite interval mapping analysis for eight quantitative traits identified 21 QTL distributed across nine linkage groups. Of these, QTL on linkage groups A02, A06, C01 and C02 overlapped with resistance QTL for control of two different diseases (phoma stem canker and light leaf spot) found in previous studies that used the same DY DH mapping population, suggesting that these QTL may be stable. Qualitative assessment of black necrotic flecking in different environments, on different cultivars and on different areas of the leaf suggested that this phenotype may be associated with disease pressure and may be isolate-specific, but its underlying mechanisms remain unknown. This project improved understanding of host resistance against P. brassicae within oilseed rape by confirming existing knowledge and presenting novel findings that can be applied for light leaf spot disease management.