Plant Disease Complex: Antagonism and Synergism Between Pathogens of the Ascochyta Blight Complex on Pea

In a disease complex, different pathogen species cause similar symptoms on a common host plant species. These pathogens commonly develop simultaneously in a field and can then infect the same host individuals. Co-occurring pathogens may affect each other, through antagonism and/or synergism. An important question is the ecological and pathological consequences of co-occurrence of pathogenic species within a disease complex. By using two pathogens (Mycosphaerella pinodes and Phoma medicaginis var. pinodella) of the Ascochyta blight complex, this study aimed at describing how co-occurrence affects the development of pathogens and disease severity, and at defining the prerequisites for interactions between pathogens. Results showed that the presence of the two pathogens on the same host plant organ limited the disease development and their reproduction. Damages caused by the two pathogens, however, increased when plants that had been previously inoculated were inoculated with the other species. The development of one pathogen species can be affected by a subsequent pathogen inoculation. Co-occurrence of pathogens within a disease complex can then limit or favour plant damages and pathogens reproduction. It would be one of the main forces that shape pathogen community structures and therefore the dynamics of diseases in field.