Pathogenicity and Host Range of <i>Stemphylium vesicarium</i> , Causing Leaf Spot of Quinoa in Canada

ABSTRACT Leaf spot is an emerging disease in quinoa cultivation in Canada. Between June and October 2022, quinoa plants with black‐brown leaf spots were observed in fields across Saskatchewan (western Canada). This study aimed to identify the causal pathogen, characterize its morphology and phylogeny, and evaluate its pathogenicity and host range across 17 crops, including beet, canola, faba bean, field bean, lentil, pea, quinoa, red kidney bean, tomato and wheat. Forty‐seven fungal isolates were obtained from infected quinoa leaves. Morphological analysis of 15 isolates revealed medium‐to‐deep brown, oblong, muriform conidia (18.6–41.5 × 10.5–21.5 μm) with 4–9 septa. Phylogenetic analysis based on the ITS, GAPDH and cmdA loci confirmed that representative isolates from this study formed a monophyletic cluster with ex‐type and reference isolates of Stemphylium vesicarium , supported by 100% bootstrap values. Pathogenicity tests showed that infection in quinoa started as light brown spots, primarily confined to the leaf margins, progressing to necrotic lesions and tissue decay. Faba bean and lentil were highly susceptible, whereas quinoa showed notable susceptibility to isolates IMSS269, IMSS273 and IMSS282. Detached leaf assays revealed isolate IMSS263 had the broadest host range, infecting 11 of 14 tested plants. Beans and beets were highly susceptible, whereas wheat and peas were incompatible. Isolates IMSS280 and IMSS269 were the most aggressive, inducing severe necrotic lesions across quinoa cultivars, whereas IMSS263 caused mild symptoms. The pathogen was successfully reisolated from symptomatic leaves, thereby fulfilling Koch's postulates and confirming S. vesicarium as a causal agent of quinoa leaf spot.