First Report of Leaf Anthracnose on <i>Calathea orbifolia</i> Caused by <i>Colletotrichum siamense</i> in China

Peacock plant (Calathea orbifolia [Linden] H. A. Kenn.) is grown in China as a valuable houseplant. In September 2024, leaf anthracnose was found on this plant in a field in Shenzhen (22°35'55"N, 113°59'21"E), Guangdong Province, China. The area investigated was ~900 m2 and included ~300 plants. Disease incidence was ~70% of 100 plants. Disease severity per plant was 20 to 60% necrotic lesions on the leaf. The initial symptoms were brown spots with a yellow halo. The spots enlarged, elongated, irregular, 0.5 to 9 × 0.2 to 4 cm, and leaves withered and died. Ten symptomatic leaves were collected for pathogen isolation. Small pieces (5×5 mm2) of lesion margins were surface sterilized in 1% NaClO for 1 min, 70% ethanol for 30 s, rinsed in sterile distilled water, placed on PDA, and incubated at 25°C. After 3 days, fungal colonies were purified on PDA using the single hyphal tip method. Three fungal strains (MBSZU 24-004 to MBSZU 24-006) with similar morphology were obtained from different plant samples with an isolation frequency of 75%. Colonies on PDA were white, 30 to 35 mm in diameter after 3 days at 25°C, then grayish white with cottony mycelia, the reverse pale yellow after 1 week. All strains produced asexual structures. Setae were dark brown, 43.2 to 169.4 × 2.1 to 4.3 µm, a cylindrical base, and an acuminate tip. Conidiophores were septate, hyaline to pale brown, and branched. Conidiogenous cells were hyaline, cylindrical to ampulliform, 10.3 to 32.3 × 2.2 to 4.4 µm. Appressoria were oval to irregular, dark brown to black, 6.2 to 18.9 × 5.7 to 10.1 µm. Conidia were one-celled, hyaline, smooth-walled, cylindrical, ends rounded, guttulate, 9.5 to 20.6 × 3.6 to 6.2 µm (n = 100). Morphologically, all strains resembled the C. gloeosporioides species complex (Weir et al. 2012). The ITS, ACT, CAL, CHS-1, TUB2, GAPDH, and ApMat genes were amplified using primer pairs ITS5/ITS4, ACT-512F/ACT-783R, CL1C/CL2C, CHS-79F/CHS-345R, T1/T22, GDF1/GDR1, and AMF1/AMR1 (Silva et al. 2012; Weir et al. 2012), respectively. Sequences were deposited in GenBank (ITS: PQ550808 to PQ550810; ACT: PQ558623 to PQ558625; CAL: PQ558629 to PQ558631; CHS-1: PQ558632 to PQ558634; TUB2: PQ558626 to PQ558628; GAPDH: PQ558635 to PQ558637; ApMat: PQ777471 to PQ777473). Maximum likelihood phylogenetic analyses of the combined seven genes identified all strains as C. siamense. To test pathogenicity, both wounded and unwounded healthy leaves were used in this experiment. All leaves were wiped with 0.1% NaClO and then rinsed with sterile water. Conidia suspensions (15 µl of 1 × 106 conidia/ml) of all three strains grown on PDA at 25°C for 2 weeks were placed on all samples using the attached leaf assay. Control leaves were mock-inoculated with sterile distilled water. Each treatment had 10 replicates and was repeated twice. Plants were placed at 25°C and 80 to 85% relative humidity. After 10 days, both wounded and unwounded inoculated leaves exhibited brown lesions with yellow halos, while the control leaves remained asymptomatic. Wounded leaves showed more symptoms than unwounded leaves. Strain MBSZU 24-005 was more virulent than the others. C. siamense was reisolated from the lesions and confirmed by morphology and DNA sequencing, fulfilling Koch's postulates. To our knowledge, this is the first report of C. siamense causing anthracnose on C. orbifolia in China and worldwide. The disease could economically impact commercial growers, necessitating monitoring of its effects on China's peacock plant production.