First report of Leaf Spot on Welsh onions ( Allium fistulosum L.) Caused by Stemphylium vesicarium in China
In September 25, 2025, numerous symptoms resembling leaf spot were observed on the leaves of Welsh onions planted at the Beijing Academy of Agricultural and Forestry Sciences (39°57′N, 116°18′E). Samples were taken from the diseased leaves. The lesions mostly appeared at the leaf tips and middle parts of the leaves, exhibiting a yellowish-white discoloration, circular or oval shapes, slightly sunken, and in severe cases. The samples were isolated in a hood, soaked in 75% alcohol for 30 s, rinsed twice with sterile water for 5 s each time, and finally cut into leaf pieces（5 × 5 mm）. These pieces were inoculated onto Potato Dextrose Agar (PDA) medium and cultured in a dark incubator at 25°C. Two isolates (designated BJST4 and BJST6) were isolated. Mycelia were white and irregularly floccular at the initial stage. The medium gradually turned yellow, later, they turned cottony-woolly. The medium gradually turned reddish-brown. Conidia are pale yellowish-brown, elliptical, and muriform with transverse septa, constricted at the central septum. After 10 d, the mycelia were picked and DNA was extracted using the CTAB method. Primers ITS1/ITS4 were used to amplify the Internal Transcribed Spacer (ITS) regions (White et al., 1990), and primers gpd1/gpd2 were used to amplify the glyceraldehyde-3-phosphate dehydrogenase (gpd) gene (BerBee et al., 1999). The amplification products were sent to Tsingke BiotechnologyCo., Ltd. for sequencing. BLASTn analysis revealed that the ITS sequences of both isolates were 100% homology to Stemphylium vesicarium (YLXXC10-21 and YLXXC08-9), and the gpd sequences showed over 99% homology to multiple S. vesicarium. MEGA7 was used to align and trim the obtained sequences, and phylogenetic trees were constructed for the ITS and gpd sequences separately. The ITS and gpd sequences have been submitted to GenBank (GenBank accession nos. PZ169639, PZ169640, PZ179494, and PZ179495, respectively). To verify Koch's postulates, inoculation was performed on detached Welsh onion leaves. The leaves were cleaned with 75% alcohol and rinsed with sterile water. The edge hyphae of a 5-day-old strain were inoculated onto the pricked and wounded leaves. The control group used PDA medium without inoculation. The inoculated leaves were placed in a light incubator (humidity 99%, temperature 25°C with a 12-h photoperiod). On the third day of inoculation, disease spots appeared in the treatment, while the control group remained healthy. The strain re-isolated from the lesions was identical to the original inoculated strain. To our knowledge, this is the first report of leaf spot disease caused by S. vesicarium on Welsh onions in Beijing, China. This discovery provides an effective direction for the identification and treatment of Welsh onion diseases in production.