Occurrence of Stemphylium leaf spot caused by Stemphylium vesicarium in commercial celery production following an onion rotation

In 2022, lesions were observed on the lower leaves of ‘Fandango’ celery (Apium graveolens L) growing in a commercial field located in Hamilton, MI. Brown spots were observed on the leaves’ adaxial surface and border extending to the petiole. Conidia were observed on the symptomatic tissue and 25 fungal isolates, morphologically similar to S. vesicarium, were obtained. DNA was extracted from three isolates and two primer sets were used to amplify and sequence the internal transcribed spacer (ITS) region and partial calmodulin (cmdA) and glyceraldehyde-3-phosphate dehydrogenase (gapdh) genes. The obtained sequences of the three isolates had 100% pairwise identity with S. vesicarium sequences MW798751 (ITS), MK675706 (cmdA) and OQ925923 (gapdh). A multilocus phylogenetic analysis (neighbor-joining tree) strongly supported clustering Michigan isolates into a single clade with S. vesicarium reference sequences. Isolates recovered from celery were pathogenic to ‘CR1’, ‘Tall Utah’ and ‘Challenger’ celery and ‘Bradley’ onion. S. vesicarium isolates isolated from symptomatic onion volunteers were pathogenic to the three celery cultivars. To our knowledge, this is the first report of Stemphylium vesicarium as a pathogen on celery causing leaf spot, and volunteer onion may be a source of Stemphylium in celery fields.