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

Abstract

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.

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“…Studies showed that the maximum damage to onion stemphiliosis was observed in 2021, which was characterised by high humidity, heavy precipitation, and elevated air temperature. These results are consistent with previous experiments conducted by various authors using onion cultures (Wang et al, 2021;Wright et al, 2019). Gossen et al (2021) identified that the first symptoms of onion stemphiliosis coincided with the appearance of a large number of conidia, precipitation, leaf moisture duration of ≥8 hours, and days with an average daily temperature of ≥18°C.…”

Diversity and Biocontrol Potential of Endophytic Fungi and Bacteria Associated with Healthy Welsh Onion Leaves in Taiwan

“…Pathogenic fungal strains, Colletotrichum spaethianum strain SX15-2 and Stemphylium vesicarium strain SX20-2, were isolated during our previous studies from infected Welsh onion plants with leaf blight symptoms [ 6 ]. For inoculation experiments, the healthy and mature Welsh onion seedlings (70–90 days after planting) were obtained from Welsh onion fields in Wan-Fu Village, Sanxing, Taiwan.…”

“…Control plates were prepared with only the pathogen. The inhibition rate of mycelial growth (IRM) was evaluated using the formula below [ 6 ]: IRM (%) = (Control colony diameter − Treatment colony diameter/Control colony diameter) × 100% …”

“…One of the major Welsh onion-growing areas in Taiwan is Sanxing Township, in Yilan County, and Welsh onion in Sanxing is famous for the unique flavour of Welsh onion cultivar grown in this area named ‘Si-Ji-Cong’. However, Welsh onion cultivation in Sanxing is severely affected by two major foliar pathogenic fungi: Stemphylium vesicarium and Colletotrichum spp., causing Stemphylium leaf blight (SLB) and anthracnose, respectively [ 6 ]. The current management of foliar diseases of Welsh onion mainly relies on fungicides in Taiwan.…”

“…Fungicides such as QoIs (Quinone outside inhibitors), SDHIs (Succinate dehydrogenase inhibitors), demethylation inhibitors, and dicarboximide are widely used on Allium crops in Sanxing to control foliar diseases, but the amount of fungicide application has continuously increased over time [ 6 ]. In a recent study, Wang et al reported that foliar pathogens, especially S. vesicarium strains, showed resistance to fungicides such as strobilurin plus azoxystrobin and kresoxim-methyl that have been used to control SLB in Taiwan [ 6 ]. This scenario has been observed in many other countries including the USA and Canada [ 7 ].…”