Occurrence of date palm ( Phoenix dactylifera L.) anthracnose caused by Colletotrichum siamense in China

Date palm (Phoenix dactylifera L.) is a major crop in the Middle East and North Africa (Bouhssini, 2018). In recent years, multiple varieties were introduced to China. In December 2024, A anthracnose incidence of 27% was recorded among 100 date palms surveyed in orchards in Wenchang, China (19.543745°N, 110.77291°E). The typical symptoms included small yellow-brown spots with purplish-red margins, enlarging to irregular brown lesions with grayish-white centers and black acervuli. Twelve diseased leaves from five plants were collected. Lesion pieces (5 mm) were surface-sterilized with 75% ethanol (15 s) and 0.1% HgCl2 (3 min), rinsed thrice in sterile water, then cultured on PDA at 25℃ for 2 days. Eight isolates with consistent morphology were obtained via hyphal-tip purification (Leslie and Summerell, 2006). Representative strain 24-1227 was selected. The colonies were grayish-white with a fluffy mycelium, gray-green centers, and a brownish reverse side. Conidia were hyaline, unicellular, and falcate to cylindrical in shape, 6.1 to 16.7 (x̄ = 11.3) × 3.0 to 5.9 (x̄ = 4.3) µm. Morphological features are insufficient to differentiate cryptic species in the Colletotrichum complex; multilocus phylogenetic analysis was thus performed to confirm the species identity of the isolate. For molecular identification, genomic DNA was extracted (Bacterial Genomic DNA Extraction Kit), and the ITS (White et al., 1990), ACT (Carbone and Kohn,1999), HIS (Crous et al., 2004), CHS (Carbone and Kohn,1999), and GAPDH (Guerber et al., 2003) sequences amplified by PCR. The sequences were deposited under accession Nos. PX780739 (ITS), PZ631774 (ACT), PZ631775 (HIS), PZ631776 (CHS) and PX833603 (GAPDH). BLASTn showed 100% (ITS), 99.65% (ACT), 99.27% (HIS), 98.34% (CHS) and 98.21% (GAPDH) similarities to Colletotrichum siamense strains KC297065, KC296929, KC297043, KC296984, KC297007, respectively. Phylogenetic analysis based on the concatenated sequence alignment showed that the isolate 24-1227 clustered within the same subclade with C. siamense strains from GenBank with a 94 % bootstrap value. Based on morphology and phylogenetic analyses, the isolate was identified as C. siamense. Pathogenicity of the fungus was tested by inoculating 10, 3-month-old tissue culture seedlings of P. dactylifera. Agar plugs (5 mm in diameter) from the periphery of 7 day old 24-1227 colonies grown on PDA were placed on surface-sterilized leaf needle-wound sites; the experiment was repeated three times. Controls received plain PDA discs. The inoculated seedlings were kept at 25℃ and 80% relative humidity, with a 12-h light/dark photoperiod. Seven days after inoculation, symptoms observed on inoculated leaves were similar to those in the field, while the control still remained symptomless. C. siamense was recovered from the inoculated leaves but not from the control leaves, confirming Koch’s postulates. Minor morphological divergence between artificial inoculation lesions and natural field spots arises from needle wounding, but key diagnostic anthracnose characteristics and fungal sporulation structures were consistent with field infections. C. siamense causes leaf spot on silver date palm (Phoenix sylvestris Roxb.) in China (Yang et al., 2024). To our knowledge, this is the first report of C. siamense causing leaf anthracnose on P. dactylifera in China. Currently, this disease is common in planting areas and is mainly controlled through chemical agents and tree spacing.