First Report of Leaf Anthracnose Caused by Colletotrichum fructicola on Calathea orbifolia in China.

Abstract

The peacock plant (Calathea orbifolia [Linden] H. A. Kenn.) is widely grown in China as a valuable ornamental houseplant. In September 2024, anthracnose on this plant was observed in a field in Shenzhen (22°35'55"N, 113°59'21"E), Guangdong Province, China. The area investigated was ~900 m2, with a disease incidence of ~70% among 100 plants. Each plant had 20 to 50% leaf necrosis. This disease may become a greater problem as the plant's cultivation area expands. The initial symptoms were brown spots with a yellow halo that later enlarged and elongated, 0.5 to 8 × 0.3 to 5 cm. The spots were irregular and brown, with a yellow halo, and the affected leaves withered and dried. Ten symptomatic leaves were randomly collected and used to isolate the fungal causal agents through a tissue transplanting method. Three fungal isolates (MBSZU 25-019 to MBSZU 25-021) with similar morphology were obtained with an isolation frequency of 75%. Colonies on PDA were 75 to 83 mm after 1 week at 25°C, appeared grayish-white with cottony mycelia, slightly raised with entire edges, and were gray in the center and white at the margin on the reverse side. All isolates produced asexual structures. Setae were 40 to 11 × 2.4 to 4 µm, dark brown, a cylindrical base, and an acuminate tip. Conidiophores were hyaline to pale brown, septate, and branched. Conidiogenous cells were cylindrical to ampulliform, hyaline, and 8 to 24 × 2.7 to 5.5 µm. Appressoria were oval to irregular, dark brown to black, and 8 to 16.3 × 5 to 8.8 µm. Conidia were cylindrical, one-celled, hyaline, aseptate, smooth-walled, ends rounded, guttulate, and 9 to 23 × 4 to 6.5 µm (n = 50). Morphological characteristics of all isolates resembled Colletotrichum spp. (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. The ITS (PV162976 to PV162978), ACT (PV175253 to PV175255), CAL (PV175259 to PV175261), CHS-1 (PV175262 to PV175264), TUB2 (PV175256 to PV175258), GAPDH (PV175265 to PV175267), and ApMat (PV175268 to PV175270) sequences were deposited in GenBank. In the BLAST analysis, the ITS, ACT, CAL, CHS-1, TUB2, GAPDH, and ApMAT sequences showed similarities of 99.83, 99.56, 100, 99.67, 99.85, 99.29, and 90.80%, respectively, to C. fructicola (ICMP 18581). Maximum likelihood and Bayesian inference phylogenetic analyses of the concatenated seven genes identified all isolates as C. fructicola. To test pathogenicity, the healthy leaves were wiped with 0.1% NaClO and then rinsed three times with sterile water. Conidia suspensions (15 µl of 1 × 106 conidia/ml) from each isolate cultured 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 was replicated ten times and repeated twice. Plants were kept at 25°C with a relative humidity of 80 to 85%. After 7 days, all inoculated leaves displayed brown lesions with yellow halos, whereas control leaves had no symptoms. C. fructicola was consistently reisolated from the lesions and confirmed by morphology and DNA sequencing, fulfilling Koch's postulates. To our knowledge, this is the first report of leaf anthracnose on C. orbifolia caused by C. fructicola in China and worldwide. This report will help growers recognize this disease and take appropriate measures to minimize or prevent economic losses.