Interspecific Interactions and Host Background Influence the Population Dynamics of the Species Causing the Ascochyta Blight Complex in Pea

ABSTRACT Ascochyta blight (AB) is an important seed‐borne and foliar disease of field peas and is widely recognised as a major productivity barrier. The disease is caused by several necrotrophic fungal pathogens, including Didymella pisi , D . pinodes , D . pinodella and Ascochyta koolunga . These pathogens can exist independently or together (known as the Ascochyta blight disease complex) within a pea field and even on single plants. Different AB pathogens constantly interact with each other on host plants. The interspecific interactions can remarkably influence disease development and outcome. Understanding the interactions of different AB pathogens on disease development will help us better control this serious disease. In this study, we investigated the interaction of the three AB pathogens, D . pisi , D . pinodes and D . pinodella , in single pea plants. A biocontrol agent, Pseudomonas protegens (Pf‐5), was also included in this study to understand the impact of plant‐beneficial microbes on the interspecific interactions of the AB pathogens. The results revealed a dual interaction with both mutualism and competition among the AB species over time. The mutualistic interaction among the three species allowed D . pisi to establish growth in a previously immune cultivar, Fallon. Meanwhile, D . pinodes became more aggressive and reached up to 60% of the abundance in the population. These findings provide a knowledge base for effective integrated management of pea AB disease, highlighting the importance of Ascochyta species inspection during seed exchange, target species for resistance breeding, and Pseudomonas protegens Pf‐5 as a potential biocontrol agent.