First Report of Soft Rot of Pitahaya Fruit Caused by Gilbertella persicaria in Mexico.

Abstract

Pitahaya (Hylocereus spp.), also called dragon fruit, is a cultivated cactus that is native to Mexico as well as Central and South America. In September 2021, soft rot of fruit of H. ocamponis, H. undatus, and H. costaricensis was observed in a commercial orchard located in La Cruz de Elota, Sinaloa, Mexico. The disease occurred on approximately 15% of pitahaya fruit. Lesions on fruits were water-soaked and light brown, extending to the whole fruit and covered with mycelia, sporangiophores, and sporangia. Colonies of a fungus were consistently isolated on PDA medium and 10 isolates were obtained. Three isolates were selected and deposited in the Culture Collection of Phytopathogenic Fungi at the Research Center for Food and Development (Culiacán, Sinaloa) under accession nos. CCLF171-CCLF173. Colonies on PDA medium were initially white and later grayish. Sporangiophores were hyaline to light brown, and aseptate. Sporangia (n= 30) were initially light brown but became black at maturity, globose to subglobose, single, terminal, 65.8 to 117.2 µm in diameter, and longitudinally separated into two halves. Columellae (n= 20) were light brown, obovoid, 33.5 to 72.9 × 31.5 to 69.8 µm, with a distinct basal collar. Sporangiospores (n= 100) were hyaline, globose to ellipsoid, aseptate, 6.9 to 12.8 × 5.1 to 10.9 μm, with polar appendages. Chlamydospores were solitary or in chains, oval or irregular. Zygospores were not observed. Based on the morphological characters, the fungal isolates were identified as Gilbertella persicaria (Benny 1991). To confirm the identity, total DNA was extracted, and the internal transcribed spacer (ITS) region was amplified by PCR using the primers ITS5/ITS4 (White et al. 1990), and sequenced. The ITS sequences were deposited in GenBank under the accession nos. OM301904-OM301906. A BLASTn search of these sequences showed 99.47 to 99.81% identity with the sequence MK301174 of G. persicaria from Hylocereus sp. in Taiwan. A phylogenetic analysis based on Maximum Likelihood method grouped the isolates CCLF171-CCLF173 within the G. persicaria clade. Pathogenicity of the three isolates was verified on healthy Hylocereus spp. fruit. Fruit of H. ocamponis, H. undatus, and H. costaricensis were surface sterilized with 80% ethanol, and dried. For each fungal isolate, five detached fruits were superficially wounded with a sterile toothpick and inoculated by placing 15 µL of a spore suspension (1 × 105 sporangiospores/mL). Sterile distilled water was applied to five healthy pitahaya fruits to serve as controls. All fruits were kept in a moist plastic chamber at 25°C and 12 h light/dark for 6 days. All inoculated fruits developed rot 3 days after inoculation, whereas no symptoms were observed on the control fruits. The experiment was repeated twice with similar results. The fungi were consistently re-isolated from the diseased fruits, fulfilling Koch´s postulates. Gilbertella persicaria has been previously reported to cause stem rot, fruit rot, and wet rot in pitahaya (Hylocereus spp.) in Japan (Taba et al. 2011), China (Guo et al. 2012), and Taiwan (Lin et al. 2014), respectively. To our knowledge, this is the first report of G. persicaria causing soft rot of Hylocereus spp. fruit in Mexico. Additional studies are needed to develop effective disease-management strategies.