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First Report of Colletotrichum siamense Causing Leaf Spot on Redbud in China

Ji, J., Wang, T., Xu, X., Wang, X. Y., Wu, Q. Q., Li, W. Y., Kan, Y. C., Yao, L. G.
Plant disease 2019 v.103 no.3 pp. 585
Callistephus chinensis, Cercis chinensis, Colletotrichum, DNA, actin, anthracnose, chitin synthase, color, conidia, conidiomata, containers, culture media, death, droplets, ethanol, flowers, genes, internal transcribed spacers, landscapes, leaf spot, leaves, mycelium, ornamental value, pathogenicity, pathogens, phenotype, relative humidity, sodium hypochlorite, trees, wilting, Argentina, China, India
Chinese redbud (Cercis chinensis), with heart-shaped leaf lamina and purple flower, is widely planted as an important landscape species all over China owing to its ornamental value. From July to August 2015, a severe leaf-spot disease was observed in the campus of Nanyang Normal University (32.516°N, 112.329°E) located in Nanyang of Henan Province. Pointed brown specks first appeared on leaves. The spots expanded from irregular or circular (up to 57 to 210 mm in diameter) shapes with orange to pink center and dark brown borders, eventually leading to wilting and death of the leaves. Twenty infected samples were collected from several individual trees. Diseased tissue was cut into 2 × 2-mm pieces, sterilized in 75% ethanol for 30 s, soaked in 1% NaClO for 1 min, rinsed with sterilized water, and transferred onto potato dextrose agar (Oxoid) plates. These plates were incubated at 28°C for 5 days. The colonies of all the tested isolates showed similar phenotypes, cottony, greyish white colony color, and white to gray aerial mycelia. Conidia were single celled, hyaline, cylindrical, clavate, and measured 4.3 to 6.5 × 13.8 to 17.2 µm (4.92 ± 0.38 × 15.78 ± 1.97, n = 100). The conidiomata can be directly observed on lesions of the host tissue. Conidiomata were spherical, dark brown, and surrounded by several brown setae. These observed morphological and cultural features indicated that the pathogen possessed identical characteristics as those reported in a previous description of Colletotrichum siamense Prihastuti, L. Cai & K.D. Hyde (Yang et al. 2009). For further molecular identification, the internal transcribed spacer (ITS) and the genes encoding for actin (ACT) and chitin synthase (CHS) regions of isolate HN17-08 were amplified from genomic DNA. BLASTn analyses of ITS (accession no. MH370612) sequence exhibited 100% identify to C. siamense strain SH-7 (MF554904). The sequence of ACT (MH370613) and CHS (MH370614) exhibited 100 and 99% identity to the same strain SH-7 (MF554892 and MF554933, respectively) (Wang et al. 2017). To conduct a pathogenicity test, conidia suspension (10⁶ conidia/ml) was prepared. Ten healthy leaves from 2-year-old redbud trees were inoculated with four droplets of 20 µl of conidia suspension (four droplets for each leaf), whereas the control was treated with sterile water by the same method. The samples were kept in containers covered with plastic membranes with 80% relative humidity inside and were incubated at 28°C with a 12-h photoperiod. The pathogenicity tests were repeated three times. Dark brown spots similar to the field disease symptoms were observed on the inoculated leaves after 5 days, whereas no visible symptoms appeared on the uninoculated control leaves. Furthermore, from the symptomatic inoculated leaves, C. siamense strains were successfully reisolated, indicating that this species was the pathogen according to Koch’s postulates. C. siamense infects more than 60 plant species worldwide, including three reported Fabaceae species from Argentina, India, and China (Larran et al. 2015; Liu et al. 2018; Srivastava et al. 2017). To our knowledge, this is the first report of anthracnose on C. chinensis caused by C. siamense.