First Report of Anthracnose Caused by Colletotrichum karstii on Fatsia japonica in Sichuan, China

Fatsia japonica (Thunb.) Decne. & Planch. (Araliaceae), an evergreen shrub, is widely planted in urban settings at the south of the Yangtze river in China. Leaf-spot symptoms were observed on 120/200 F. japonica plants at Sichuan Agricultural University. Initially, yellowish spots appeared on the leaves and became white with age. The spots continued to expand with time developing irregular margins eventually encompassing the entire leaf. Eventually, diseased leaves became curled and died. Four single-spore isolates were obtained following Chomnunti et al. (2014). The colonies developed on potato dextrose agar (PDA) were white to grey with fluffy aerial hyphae. The base of the mycelium was yellow. On the host conidiomata circular to elliptical, measured 108-335 × 107-250 μm (n=20). Conidiophores were bell-shaped to cylindrical, hyaline, usually 1-celled, unbranched, 9.7-23.6(-28) × 3.5-5.5 μm (x̅ =16.5 × 4.6 μm, n=15). Conidia measured 12.5-17.6 × 5.1-7.9 μm (x̅ =14.9 × 6.7 μm, n=25), were hyaline, straight, unicellular, cylindrical. Appressoria formed abundantly on slides, measured 12.4-25.4(-29.6) × 6.3-18.4 μm (x̅ =18.9 × 13.4 μm, n=30), were dark brown to black, elliptical to irregular. Ascomata on PDA were brown to dark brown, global, covered by aerial mycelium, and setae were absent. Ascospores measured 13.2-19.1 × 4.0-6.1 μm (x̅ =16.0 × 4.8 μm, n=20), and one-celled, hyaline, slightly curved. The representative isolate SICAUCC 20-0010 was used for Genomic DNA extraction following the instructions of Plant Genomic DNA extraction kit^TM (Tiangen, China), and the internal transcribed spacer (ITS, MT393946), glyceraldehyde-3-phosphate dehydrogenase (GAPDH, MT425977), beta-tubulin (TUB2, MT425978), chitin synthase (CHS-1, MT425976) and actin (ACT, MT425975) genes were amplified with ITS5/ITS4 (White et al. 1990), GDF/GDR (Guerber et al. 2003), T1/Bt2b (O'Donnell et al. 1997), CHS-79F/CHS-354R and ACT-512F/ACT-783R (Carbone et al. 1999) primers, respectively. ITS, GAPDH, CHS-1 and ACT blast showed 100% homology with sequences of Colletotrichum karstii (Yang et al. 2011, Damm et al. 2012, Xu et al. 2019), viz. JQ005186 (555/555 bp), MK359214 (238/238 bp), JQ005360 (274/274 bp), MK359211 (269/269 bp), respectively. TUB2 showed 99.60% homology with sequences JQ005620 (495/497 bp). The fungus was identified as C. karstii based on morphology and a multigene phylogeny (Fig. 1). Four leaves from each of three healthy F. japonica plants were sprayed with a conidial suspension (1 × 10⁵ conidia/ml) of the isolate SICAUCC 20-0010. Two additional mock-inoculated control plants were sprayed with sterile distilled water. Plants were incubated in a greenhouse at 26℃ and 90% relative humidity with a 12-h photoperiod. The leaves developed chlorotic lesions 6 days post-inoculation and by twenty-two days post-inoculation the remaining signs and symptoms had developed. No symptoms developed on the mock-inoculated controls. The fungus isolated from inoculated plants was morphologically identical to the original pathogen, fulfilling Koch's postulates. The anthracnose on F. japonica caused by C. fructicola and C. gloeosporioide have been reported in China (Wang 2007, Shi et al. 2017). However, this is the first report of anthracnose caused by C. karstii on F. japonica. Field observations indicated that this disease mainly occurs on old and weakened leaves. This disease affects the aesthetic appearance of the plants reducing their appeal as landscape plants.

Keywords: Colletotrichum karstii; Fatsia japonica; Anthracnose; China.