First Report of Colletotrichum spaethianum Causing Anthracnose on Jiaotou (Allium chinense G. Don) in China.

Abstract

Jiaotou (Allium chinense G. Don) is a culinary and aromatic herb belonging to the family Amaryllidaceae. It is native to China and cultivated for its culinary uses in many other countries, which also possess high medicinal value in traditional Chinese medicine (Rhetso et al. 2020). In March 2023, a disease resembling anthracnose was detected in the fields of Changde city, Hunan Province, China. The symptoms manifested as yellow to brown necrotic spots located at the tip or middle of the blade. The lesions tended to wrinkle, leading to leaf distortion, thinning, and wilting. This disease has a high incidence rate of approximately 50% in an area of nearly 20 hectares. Ten symptomatic plants were collected for pathogen isolation. The leaf tissues were surface sterilized using 70% ethyl alcohol and 2% sodium hypochlorite, rinsed with sterile distilled water, and then transferred to potato dextrose agar (PDA) and incubated at 26°C in the dark. Ten colonies with similar cultural morphology were selected from the 10 affected plant samples. The fungal strains produced densely aerial mycelium on PDA, displaying off-white to gray-brown coloration. Conidia were hyaline, curved or slightly curved, aseptate, with a rounded apex and truncate base, contained oil globules, and measuring 21.0 to 29.0 × 4.1 to 5.3 μm (n = 50. Appressoria were dark brown, irregular in shape, partially lobed, and measured 6.5 to 15.5 µm long × 5.0 to 10.5 µm wide. Setae were straight, dark brown, with two- to three-septate, 78.5 to 155.4 μm long (n = 50). These morphological characteristics were similar to Colletotrichum spaethianum (Damm et al. 2009). The internal transcribed spacer (ITS) region of rDNA, actin (ACT), chitin synthase (CHS-1) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes of three representative strains, JT-1, JT-2 and JT-3, were amplified and sequenced as described in Weir et al. (2012). These sequences were 99 to 100% identical to corresponding sequences of C. spaethianum strains (565/569 bp, MN396364.1 for ITS; 565/569 bp, MN433299.1 for ACT; 262/263 bp, MN433298.1 for GAPDH; 272/272 bp, MH020773 for CHS-1, respectively) (Zhong et al. 2020; Liu et al. 2020; Sun et al. 2020). A concatenated phylogenetic tree showed that all three strains were clustered within the C. spaethianum clade. The sequences of isolate JT-1 were deposited in GenBank (accession nos. PP911453, PP911607, PP911605, and PP911606 for ITS, ACT, GAPDH and CHS-1, respectively). For pathogenicity assays, 1-month-old seedings of A. chinense in plots were inoculated by spraying with a conidial suspension (1 × 105 conidia/ml) until runoff. Control plants were sprayed with sterilized water. All the inoculated plants were kept in a greenhouse at 25°C with a 12-h/12-h light/dark cycle. The experiment was conducted twice with three replications each. After 7 days, symptoms similar to those observed in the field developed on leaves of the inoculated plants, whereas the control plants remained asymptomatic C. spaethianum was specifically reisolated and identified from the symptomatic leaves, thus fulfilling Koch's postulates. Colletotrichum spaethianum has been recorded as a pathogen fungus on many medicinal plants, such as Peucedanum praeruptorum (Guo et al. 2013), Anemarrhena asphodeloides (Okorley et al. 2019), and Paris polyphylla (Zhong et al. 2020). To our knowledge, this is the first report of C. spaethianum causing anthracnose on Jiaotou in China. Effective control measures should be implemented to mitigate potential further damage to Jiaotou cultivation.