Abstract BACKGROUND Phoma stem canker is one of the most economically damaging diseases of oilseed rape, caused by co‐existing fungal pathogens Leptosphaeria maculans ( Plenodomus lingam ) and Leptosphaeria biglobosa ( Plenodomus biglobosus ). Our previous work showed that timing of their co‐inoculation affects their interspecific interactions in vitro , and that L. biglobosa can inhibit the growth of L. maculans if it is inoculated before or up to 1 day after L. maculans . However, the effects of timing of co‐inoculation on their interspecific interactions within host plants are not fully understood. RESULTS The interactions between L. maculans and L. biglobosa were investigated in planta by inoculation with L. maculans first, followed by L. biglobosa sequentially 1, 3 or 5 days later or vice versa, or L. maculans and L. biglobosa co‐inoculated simultaneously; controls were inoculation with L. maculans only, L. biglobosa only or sterilised distilled water. The results showed that L. biglobosa can inhibit the growth of L. maculans locally if it is inoculated before or up to 1 day after L. maculans in planta . CONCLUSION The results of this study provide evidence that the time L. maculans and L. biglobosa meet and interact on their host significantly affects the interspecific competition and subsequent disease severity. Leptosphaeria biglobosa can deploy multiple interspecific competition strategies against L. maculans to have local and systemic effects on the host to outcompete L. maculans . There is a need to investigate and model the factors affecting the timing of ascospore release and subsequent interactions to develop better control strategies. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.