Co‐inoculation timing affects the interspecific interactions between phoma stem canker pathogens Leptosphaeria maculans and Leptosphaeria biglobosa

© 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0BACKGROUND: Phoma stem canker is an economically important disease of oilseed rape, caused by two co‐existing fungal pathogen species, Leptosphaeria maculans (Plenodomus lingam) and Leptosphaeria biglobosa (Plenodomus biglobosus). Leptosphaeria maculans produces a phytotoxin called sirodesmin PL. Our previous work showed that L. biglobosa has an antagonistic effect on the production of sirodesmin PL if it is simultaneously co‐inoculated with L. maculans. However, the effects of sequential co‐inoculation on interspecific interactions between the two pathogens are not understood. RESULTS: The interactions between L. maculans and L. biglobosa were investigated in liquid culture by inoculation with L. maculans first, followed by L. biglobosa sequentially at 1, 3, 5 or 7 days later and vice versa; the controls were inoculated with L. maculans only, L. biglobosa only, or L. maculans and L. biglobosa simultaneously. The results showed that L. biglobosa inhibited the growth of L. maculans, the production of both sirodesmin PL and its precursors if L. biglobosa was inoculated before, or simultaneously with, L. maculans. However, the antagonistic effects of L. biglobosa were lost if it was co‐inoculated 5 or 7 days after L. maculans. CONCLUSION: For the first time, the results of this study provided evidence that the timing when L. maculans and L. biglobosa meet significantly influences the outcome of the interspecific competition between them. Leptosphaeria biglobosa can inhibit the production of sirodesmin PL and the growth of L. maculans if it is inoculated before L. maculans or less than 3 days after L. maculans in liquid culture. There is a need to further investigate the timing of co‐inoculation on interactions between L. maculans and L. biglobosa in their host plants for improving the control of phoma stem canker. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.Peer reviewe

Leptosphaeria biglobosa inhibits the production of sirodesmin PL by L. maculans

© 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/BACKGROUND: Phoma stem canker is caused by two coexisting pathogens, Leptosphaeria maculans and L. biglobosa. They coexist because of their temporal and spatial separations, which are associated with the differences in timing of their ascospore release. L. maculans produces sirodesmin PL, while L. biglobosa does not. However, their interaction/coexistence in terms of secondary metabolite production is not understood. RESULTS: Secondary metabolites were extracted from liquid cultures, L. maculans only (Lm only), L. biglobosa only (Lb only), L. maculans and L. biglobosa simultaneously (Lm&Lb) or sequentially 7 days later (Lm+Lb). Sirodesmin PL or its precursors were identified in extracts from ‘Lm only’ and ‘Lm+Lb’, but not from ‘Lm&Lb’. Metabolites from ‘Lb only’, ‘Lm&Lb’ or ‘Lm+Lb’ caused significant reductions in L. maculans colony area. However, only the metabolites containing sirodesmin PL caused a significant reduction to L. biglobosa colony area. When oilseed rape cotyledons were inoculated with conidia of ‘Lm only’, ‘Lb only’ or ‘Lm&Lb’, ‘Lm only’ produced large gray lesions, while ‘Lm&Lb’ produced small dark lesions similar to lesions caused by ‘Lb only’. Sirodesmin PL was found only in the plant extracts from ‘Lm only’. These results suggest that L. biglobosa prevents the production of sirodesmin PL and its precursors by L. maculans when they grow simultaneously in vitro or in planta. CONCLUSION: For the first time, L. biglobosa has been shown to inhibit the production of sirodesmin PL by L. maculans when interacting simultaneously with L. maculans either in vitro or in planta. This antagonistic effect of interspecific interaction may affect their coexistence and subsequent disease progression and management.Peer reviewe