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Stemphylium Leaf Blight of Welsh onion (Allium fistulosum): An Emerging Disease in Sanxing, Taiwan.

Welsh onion (Allium fistulosum L.) is one of the main and oldest vegetable crops grown in Taiwan. A severe epidemic of leaf blight in Welsh onion caused by a Stemphylium-like pathogen was found in Sanxing, Taiwan, from 2018 to 2020. However, correct species identification, biology and control of Stemphylium leaf blight (SLB) of Welsh onion is not well established. Therefore, the main objective of this study was to investigate the causal agent of SLB in Sanxing and evaluate the in vitro sensitivity of Stemphylium-like pathogen to commonly used fungicides. Phylogenetic analysis based on combining the internal transcribed spacer (ITS) region, and glyceraldedyhe-3-phosphate dehydrogenase (gapdh) and calmodulin (cmdA) gene sequences together with morphological features, identified S. vesicarium as associated with SLB in Sanxing. When inoculated onto Welsh onion leaves, the isolates caused symptoms identical to those seen in the field and S. vesicarium was reisolated, confirming Koch's postulates. We observed a higher incidence of SLB symptoms on the oldest leaves compared with younger leaves. The maximum and minimum temperatures for in vitro mycelial growth and conidial germination (%) of S. vesicarium were 20 to 30°C and 5°C respectively. Sixteen fungicides were tested for their effectiveness to reduce the mycelial growth and conidial germination of S. vesicarium in vitro. Boscalid + pyraclostrobin, fluopyram, fluxapyroxad, fluxapyroxad + pyraclostrobin were highly effective at reducing mycelial growth and conidial germination in S. vesicarium. However, strobilurin fungicides (azoxystrobin and kresoxim-methyl) commonly used in Welsh onion production in Sanxing were ineffective. This study discusses the emergence of SLB caused by S. vesicarium in the foliar disease complex affecting Welsh onion and the management of the disease using fungicides with different modes of action in Taiwan. The research will underpin the sustainable management of SLB in Sanxing, Taiwan, however further field assessments of the fungicides are warranted.