A first report Colletotrichum queenslandicum causing Anthracnose on Date palm ( Phoenix dactylifera L.) in Wenchang, Hainan, China

Date palm (Phoenix dactylifera) is an economically important crop in across arid and semi-arid regions of North Africa, the Middle East, and tropical South Asia. In 2019, China and the United Arab Emirates reached a key cooperation consensus, with the UAE donating 100,000 date palm seedlings to support China’s trial cultivation and industry research. Since then, trial planting areas have expanded from Hainan to Yunnan, Fujian, Sichuan, and other dry hot river valley areas. In December 2024, leaf spot symptoms were observed in approximately 35% of plants across a 7-ha date palm plantation in Wenchang, Hainan, China (19.553195N, 110.787465E). Typical symptoms included irregular brown lesions (5-15 mm diameter) surrounded yellow halos, which progressed to tissue necrosis. Tissue fragments (5×5 mm) were excised from lesion margins, then surface-sterilized by sequential treatment with 75% ethanol for 15 s, 0.1% mercuric chloride solution for 3 min, followed by three rinses with sterile water. Sterilized tissues were placed on potato dextrose agar (PDA) and incubated at 25℃ for 2 days. After purification via the hyphal-tip method (Leslie and Summerell 2006), colonies with dense, cottony, gray-white aerial mycelium were obtained. Conidia were hyaline, aseptate, cylindrical, measuring 7.4-18.6 × 2.5-6.8 μm (n=100), consistent with morphological characteristics of Colletotrichum spp. For molecular identification, genomic DNA was extracted from isolate 22-1227. Three loci: actin (ACT), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and internal transcribed spacer (ITS), using the primer pairs ACT-512F/ACT-783R (Carbone et al. 1999), GDF1/GDR1 (Guerber et al. 2003), and ITS1/ITS4 (White et al. 1990), respectively. Sequences were submitted to GenBank (accession numbers: ACT: PX994300; GAPDH: PX994301; ITS: PX904623). The amplicon sizes of ACT, GAPDH, and ITS were 283 bp, 277 bp and 559 bp, respectively. BLASTn analysis revealed 99.26% (ACT), 99.27% (GAPDH) and 99.28% (ITS) similarity with reference strains of Colletotrichum queenslandicum (MK948797, MK948840, MK955384). Phylogenetic analysis using MEGA 6, based on the combined ACT-GAPDH-ITS sequences, clustered isolate 22-1227 with C. queenslandicum reference strains with a 99% bootstrap value. Morphology and molecular data confirmed the isolate as C. queenslandicum. Pathogenicity was tested on 3-month-old tissue cultured date palm seedlings. Mycelial plugs (5 mm diameter) from 7-day-old PDA cultures of isolated 22-1227 were placed on needle-wound leaf sites; Sterile PDA plugs were served as controls. Each treatment included 10 replicate seedlings with 3 inoculation sites per seedling. All seedlings were incubated at 25 ℃ with 80% relative humidity under a 12-h light/dark cycle. After 7 days, inoculated leaves developed symptoms identical to those observed in the field, while controls leaves remained asymptomatic. The pathogen was reisolated from inoculated lesions and confirmed as C. queenslandicum via morphological and molecular methods, fulfilling Koch’s postulates. C. queenslandicum has been reported on Jatropha curcas (Li et al. 2024), Capsicum annuum (Romjon et al. 2024), Citrus spp. (Shidiq et al. 2024), but this is the first report of it causing anthracnose on date palm in China. With expanding date palm cultivation in China’s dry-hot river valleys, this disease may threaten to tropical fruit production, highlighting the need for further management research.