First Report of Colletotrichum aenigma Causing Anthracnose on Dragon Fruit in Korea.

Abstract

Dragon fruit (Selenicereus undatus) is a valuable fruit crop in tropical and subtropical regions. It is renowned for its nutritional benefits, such as high sodium, potassium, and vitamin levels, and as a source of prebiotics and antioxidants (Balendres et al. 2019). In July 2023, anthracnose symptoms on stems were detected on dragon fruit plants in Jeju, South Korea. The typical anthracnose symptoms, such as sunken necrotic lesions (5-20 mm in diameter), were seen on the mature stems. The disease incidence ranged from 10% to 12% among the three surveyed greenhouses. To isolate the causative organism, infected stem samples were surface sterilized, cut into small pieces, and placed on potato dextrose agar (PDA). After two days of incubation at 24ºC, white hyphae appeared on the PDA around the plant tissues. Isolates CNU H23009 and CNU H23010 were purified from a single hypha under a stereoscope (e-Xtra Figure 1). Conidial morphology was examined from two-day-old fungal cultures grown on V8 juice agar. The conidia were transparent, aseptate, cylindrical to clavate, with a rounded apex and base, and measured 11.9 - 16.85 × 5.17 - 6.91 μm (mean = 15.28 × 5.93 μm, n = 30). No appressoria was observed. Morphological characteristics indicated the isolates were Colletotrichum sp. matching the description of the C. gloeosporioides species complex (Weir et al. 2012). To further identify the isolates, genomic DNA was extracted and the ribosomal internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and actin (ACT) were amplified using ITS1/ITS4, GDF/GDR, and ACT-512F/ACT-783R, respectively (Weir et al. 2012). Based on phylogenetic analysis, the isolates clustered with C. aenigma (strains ICMP18608, ICMP18686, CSH2, and QSG1), with 71% bootstrap support, as determined using the maximum parsimony method in PAUP 4.0 (e-Xtra Figure 2). Based on morphological and molecular characteristics, isolates were identified as C. aenigma. Sequences of CNU H23009 and CNU H23010 were deposited in GenBank with accession numbers OR535144 and OR535145 for ITS, OR540725 and OR540726 for GAPDH, and OR540723 and OR540724 for ACT. The pathogenicity was tested on healthy dragon fruit stems using wound inoculation with mycelial plugs of the CNU H23009 isolate. Controls were inoculated with PDA plugs. The plants were covered with plastic bags to maintain humidity and incubated in a greenhouse at 25ºC. After two days, necrotic spots had developed on the inoculated tissues; after four days, black, irregular, and sunken necrotic lesions similar to those seen in the field were observed. No symptoms occurred in the controls. C. aenigma was re-isolated from the artificially inoculated plants and re-identified based on conidial morphology. The pathogenicity test was repeated three times with three replications for each treatment. Previous studies have reported that C. aenigma, C. gloeosoporioides, C. siamense, C. truncatum, and C. karsti cause anthracnose in dragon fruit. However, C. aenigma has been reported only in Thailand (Balendres et al. 2019; Meetum et al. 2015). To our knowledge, this is the first report of C. aenigma causing anthracnose in dragon fruit in Korea.