Understanding the Interactions between Phoma Stem Canker (Leptosphaeria maculans and L. biglobosa) and Light Leaf Spot (Pyrenopeziza brassicae) Pathogens of Oilseed Rape (Brassica napus)
The aim of this project was to understand the interspecific interactions between Leptosphaeria maculans, L. biglobosa and Pyrenopeziza brassicae in oilseed rape through identification of interactions at different levels of investigation. The aim was achieved by (1) investigating the interactions between phoma stem canker (L. maculans and L. biglobosa) and light leaf spot (P. brassicae) causal pathogens in vitro and in planta; (2) examining the interactions between L. maculans and P. brassicae on different cultivars under field conditions; (3) identifying relationships between weather, prevalence of light leaf spot or phoma leaf spot and stem canker and yield loss. Results of this project show that both direct and indirect interspecific interactions exist between L. maculans, L. biglobosa and P. brassicae. The results of in vitro experiments showed that L. maculans produces a phytotoxin called sirodesmin PL when cultured on its own, whereas when L. biglobosa or P. brassicae were cultured on their own sirodesmin PL was not produced. When secondary metabolite extracts from liquid culture filtrates from these three pathogens cultured on their own were applied to fungal plugs of L. maculans, L. biglobosa or P. brassicae, the only extract that reduced colony area was the secondary metabolite extract from L. maculans applied to L. biglobosa and P. brassicae. However, when the culture filtrate from L. maculans and L. biglobosa simultaneously cultured together was applied, there was no reduction in colony area of L. biglobosa or P. brassicae, nor was sirodesmin PL identified in the secondary metabolite extracts. Interestingly, when L. maculans and L. biglobosa were sequentially cultured 7 days apart, the secondary metabolite extract decreased the colony area of both L. biglobosa and P. brassicae, and contained sirodesmin PL. This implies that Sirodesmin PL had an inhibitory effect on L. biglobosa and P. brassicae. Therefore, these experiments showed that direct interspecific interactions exist between these three pathogens, and that direct interactions when Leptosphaeria spp. are simultaneously co-inoculated indirectly influence P. brassicae due to inhibition of Sirodesmin PL production that would not inhibit P. brassicae growth. A similar pattern was observed in planta when oilseed rape cotyledons were inoculated with L. maculans, L. biglobosa or a mixture of both Leptosphaeria spp. Large, undefined lesions developed when L. maculans conidia were applied and smaller, more well-defined dark lesions developed when L. biglobosa conidia were applied. However, when a mixture of both Leptosphaeria spp. were applied, the phenotype of the lesion was more similar to that of L. biglobosa rather than L. maculans, suggesting that L. biglobosa outcompeted L. maculans in a simultaneous infection situation. When secondary metabolite extractions were analysed, sirodesmin PL was found only in the L. maculans only treatment, not in the L. biglobosa or the Leptosphaeria spp. mixture treatments. These findings show that the interspecific interactions found in vitro were also found in planta. Additionally, in planta work using near-isogenic oilseed rape lines with or without the Rlm7 gene provided preliminary data to suggest that the presence of Rlm7 may increase the susceptibility of plants to L. biglobosa. The field experiments showed that cultivar resistance and fungicides were effective at reducing phoma stem cankers and P. brassicae sporulation as well as reducing the transmission of Leptosphaeria spp. inoculum between seasons. The monitoring of ascospore release events showed that there were differences in release timing and relative quantities of inoculum between seasons, including the simultaneous release of Leptosphaeria spp. ascospores with P. brassicae ascospores. Additionally, due to the long asymptomatic latent period between P. brassicae infection and presence of indicative light leaf spot disease symptoms, such as P. brassicae sporulation, there is no defined threshold for fungicide application to control P. brassicae. So, autumn disease control for phoma stem canker and light leaf spot causal pathogens is often defined by when the L. maculans fungicide threshold is met, irrespective of L. biglobosa or P. brassicae ascospore release events. In England and Wales, analysis of relationships between weather and light leaf spot disease prevalence or yield loss found the prevalence of P. brassicae pod lesions had positive relationships with autumn or winter temperature and a negative relationship with the number of autumn or winter air frosts. This suggests that warmer temperatures and fewer air frosts would result in a greater prevalence of P. brassicae pod lesions. There was also a positive correlation between average precipitation or mean number of rain-days in winter and spring and incidence of light leaf spot pod lesions. Met Office weather data has shown that the 5-year mean temperature average since 1969 for autumn and winter has increased whereas spring precipitation has not changed. Therefore, if this trend continues with autumn and winter getting warmer, this will increase the probability of P. brassicae infection. Therefore, findings from all experiments in this study suggest that; • Changes in Leptosphaeria spp. ascospore release patterns under natural conditions influence direct interspecific interactions between L. maculans and L. biglobosa, which indirectly affect P. brassicae. • The widespread adoption of effective integrated L. maculans control strategies under P. brassicae favourable weather conditions may unintentionally make the UK oilseed rape crops more vulnerable to P. brassicae infection. • Development of integrated pest management strategies is required to improve the control of L. maculans, L. biglobosa and P. brassicae together rather than in isolation.
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