First Report of Colletotrichum truncatum Causing Anthracnose on Melon (Cucumis melo L.) in China.

Abstract

Melon (Cucumis melo L.) is an important economic crop in China, with a planting area of about 500,000 hectares, ranking first in the world. In May 2024, anthracnose symptoms were found on melon plants, particularly severe on the mature fruits, in Binyang County, Guangxi, China. Disease incidence was between 30% to 60% in four surveyed planting areas. The symptoms on fruits initially appeared as water-soaked lesions, gradually turning into dark brown sunken lesions, sometimes with cracks. Additionally brown necrotic lesions with yellowish edges appeared on the leaves. For pathogen isolation, lesion edge tissues (3×3 mm) of fruits were surface-sterilized in 75% ethanol (30 s) and 1% sodium hypochlorite (1 min), rinsed in sterile distilled water, and plated on potato dextrose agar (PDA) amended with streptomycin sulphate (30 mg/l) for 4 days at 28°C in the dark. Ten pure isolates with similar morphology were obtained by transferring hyphal tips to new PDA plates. Colonies were round with smooth margins. Mycelium was sparse, initially pale gray, then changed to dark gray with numerous black microsclerotia after 14 days and generated a small amount of orange conidial masses afer 30 days of cultivation. Conidia were single-celled, hyaline, slightly curved, tapered tip and truncate base, with an oil globule at center, and 18.9 to 22.2 × 3.2 to 4.7 μm (n = 50). Setae initiated from an acervuli, were dark brown, septate, straight, pointed, and measuring 85.5 to 146.3 × 4.2 to 5.5 μm. Appressoria were light brown, elliptic to claviform or slightly lobed. Morphological characters were similar to Colletotrichum truncatum (Damm et al. 2009). Two representative isolates M1 and M3 were used for molecular identification. The partial internal transcribed spacer (ITS) region, actin (ACT), β-tubulin (TUB2), chitin synthase (CHS-1), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes were amplified with ITS1/ITS4, ACT512F/ACT783R, BT2A/BT2B, CHS-79F/CHS-345R, and GDF1/GDR1 primers, respectively. Sequences were deposited in GenBank (ITS: PQ549938, PQ549939; Actin: PQ562860, PQ562861; TUB2: PQ562866, PQ562867; CHS-1: PQ562862, PQ562863; GAPDH: PQ562864, PQ562865) and showed 97% to 100% similarity with C. truncatum strains. A maximum likelihood phylogenetic tree based on the concatenated these five loci in MEGA-X showed the clustering of the isolates M1 and M3 in the C. truncatum clade. Pathogenicity tests were performed twice in a greenhouse at 25 to 30°C with 90% relative humidity. The healthy living fruits were slightly wounded by sterilized needle. Then spore suspension (106 conidia/ml) of isolates M1 and M3 were inoculated onto the wounds (10 μl/wound). For each isolate, five fruits were inoculated. Control fruits were treated with sterile water. After 7 days, all the inoculated fruits showed brown lesions resembling natural symptoms, whereas no symptoms appeared on the negative controls. The same fungus was re-isolated from the symptomatic fruits, thus completing Koch's postulates. Based on morphological and molecular characteristics and a pathogenicity test, the pathogen was identified as C. truncatum. Previously, C. truncatum was reported to cause melon anthracnose in Brazil (Assunção et al. 2024) and watermenlon anthracnose in China (Guo et al. 2022). To our knowledge, this is the first report of C. truncatum causing anthracnose on melon in China. Knowing the causal agent is important to control this disease effectively.