Pathogenicity Validation of Parastagonospora pseudonodorum on Wheat and Barley

Parastagonospora pseudonodorum , previously known as Phaeosphaeria avenaria f. sp. tritici (Pat1), is a fungal species within the Septoria blotch complex that causes yield losses in cereal crops worldwide. First reported in Canada in 1942 under an earlier taxonomic name. However, successive taxonomic changes in addition to its close resemblance to Parastagonospora nodorum , the causal agent of Septoria nodorum blotch have complicated its identification and its role in pathogenicity. Recent molecular identification of P. pseudonodorum in Western Canadian wheat samples necessitated the re-evaluation of its pathogenicity and raised concerns about potential misidentification with the closely-related species P. nodorum. These two species secrete combinations of various necrotic effectors (NEs) that contribute to their virulence. In this study, we examined the pathogenicity of P. pseudonodorum following Koch’s postulates. Our results confirmed the ability of P. pseudonodorum to infect the leaves and spikes of both hosts, but the symptoms was weaker on the leaves of both hosts and wheat spikes, whereas more evident damage was observed on barley spikes. While a limited number of genotypes was tested as a proof of concept, but the observed differences in symptom severity suggest host and tissue associated variation in susceptibility. These results highlight the need to expand our monitoring of the leaf spot complex to include this species and to further explore its host range and genomic characterization. Awareness of this pathogen is essential, particularly given its association with cereal spikes and its quarantine status in some countries which could impacts the export of Canadian grains overseas.