P-1: Leptosphaeria biglobosa inhibits production of secondary metabolite sirodesmin PL by L. maculans in planta

Abstract: Phoma stem canker is an economically damaging disease of oilseed rape (Brassica
napus) in the UK, caused by two co-existing fungal pathogens Leptosphaeria maculans and L. biglobosa. Interactions between these pathogens affect disease severity. L. maculans produces a non-host selective epipolythiodioxopiperazine (ETP) phytotoxin called sirodesmin PL, allowing the pathogen to have a competitive advantage over other pathogens. L. biglobosa does not produce sirodesmin PL. Results of our recent in vitro studies indicated that L. biglobosa could inhibit production of sirodesmin PL, if co-inoculated simultaneously. In this study, we aimed to determine whether this inhibition also occurred in planta. Cotyledons of oilseed rape cultivar Charger were inoculated with L. maculans only, L. biglobosa only, L. maculans and L. biglobosa simultaneously, and sterilised distilled water as a control. Lesion phenotypes were assessed, and secondary metabolites were extracted from lesions at 26 days post inoculation and analysed by HPLC and LC-MS. L. maculans only treatment had significantly larger lesions compared to all other treatments. Lesions from L. biglobosa only and L. maculans and L. biglobosa simultaneously treatments did not have significantly different sizes. Presence of sirodesmin PL was only detected in the L. maculans only treatment. These results indicate that L. biglobosa inhibits production of sirodesmin PL by L. maculans both in vitro and in planta if the pathogens are co-inoculated simultaneously. There is a need to further investigate the mechanisms of this inhibition by L. biglobosa. Understanding interactions between L. maculans and L. biglobosa can provide new strategies for effective phoma stem canker control, alongside resistant cultivars and fungicide applications.