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Pathogen Identification and Fungicide Screening of Onion Leaf Curling Disease in the Kaohsiung-Pingtung Region
Fig. 1. 14-day-old colonies of collected isolates grown on PDA (potato dextrose agar) at 28℃. (A) 8321, (B) 8322, (C) 8323, (D) 8324, (E) 9441, (F) 9442, (G) 9443, (H) 9444, (I) 9445, (J) 9461, (K) 9462, (L) 9463, and (M) 9464.
Fig. 1. 14-day-old colonies of collected isolates grown on PDA (potato dextrose agar) at 28℃. (A) 8321, (B) 8322, (C) 8323, (D) 8324, (E) 9441, (F) 9442, (G) 9443, (H) 9444, (I) 9445, (J) 9461, (K) 9462, (L) 9463, and (M) 9464.

Author:Cheng-En Chen*、Shih-Zhu Jiang、Min-Nan Tseng

Abstract:

Onion (Allium cepa) is primarily cultivated in the Linyuan District of Kaohsiung City and the Hengchun Peninsula of Pingtung County, serving as an important winter crop in southern Taiwan. Common diseases during the growing season include Fusarium wilt and bacterial soft rot. Since 2019, onion plants showing leaf curling symptoms have been observed at the early growth stage in fields in Kaohsiung and Pingtung. Although the affected plants gradually wilted, the symptoms differed from those of typical Fusarium wilt. Symptomatic samples were collected from Linyuan, Checheng, and Hengchun, and fungal isolates were obtained. Based on fungal morphology and molecular identification, the isolates were identified as Colletotrichum spp. Phylogenetic analysis using partial sequences of the internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), β-tubulin (TUB2), and chitin synthase (CHS-1) genes indicated that the isolates were most closely related to C. siamense. Pathogenicity tests were conducted with four isolates from Linyuan, two from Checheng, and two from Hengchun. Onion seedlings were drenched with conidial suspensions at concentrations of 1 × 105 and 1 × 106 spores/mL (1 mL per seedling) three weeks after transplanting. Leaf distortion symptoms were observed 21 d post inoculation, and the incidence of distorted leaves reached up to 70% at 35 d post inoculation. Colletotrichum spp. were successfully re-isolated from the basal leaf tissues of symptomatic plants. Furthermore, twelve chemical fungicides were tested for their inhibitory effects on mycelial growth and spore germination. Among them, cyprodinil + fludioxonil and difenoconazole consistently showed strong inhibitory effects. These two fungicides are recommended as priority options for onion disease management in the Kaohsiung-Pingtung region.

Key words:Onion, Colletotrichum spp., Fungicide screening

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Journal of
Taiwan Agricultural Research
Vol. 75, No.3 (2026) ISSN: 2790-086X Vol. 75, No.3 (2026)
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