First Report of Gilbertella persicaria causing Postharvest Soft Rot of Pyrus pyrifolia 'Housui' in China.

ABSTRACT

Postharvest diseases lead to substantial economic losses to the pear industry (Xu et al. 2021). In August 2022 and 2023, 'Housui' pears (Pyrus pyrifolia) with no visible wounds were harvested from Baoying county, Jiangsu Province, China and stored at 20°C with 85% relative humidity. Approximately 8% of pear fruits showed soft rot after 15 days of storage. The margin area of rot tissue was aseptically incubated on PDA medium at 25°C. Mycelial tips were transferred to new PDA after 24 h. Five fungal isolates were obtained after isolation and identification, including Alternaria sp., Botryosphaeria sp., Diaporthe sp., Fusarium sp. and Gilbertella sp. For each isolate, pathogenicity tests were confirmed three times by placing 10 µL of spore suspension (106 spores/mL) on three 'Housui' pear fruits superficially wounded with sterile toothpicks, and sterile distilled water served as controls. Lesions caused by Gilbertella sp. were distinctly observed after incubating at 20°C for 24h, and controls have no symptom. The lesions expanded to large brown spots with smelling of alcohol after 48 h, similar to natural disease symptom. The colony of Gilbertella sp. was initially white and rapidly turned gray, generating large amounts of black sporangia. -Sporangia were firstly white, then turn black, globose to dorsoventrally flattened, 70.22 to 131.58 × 75 to 135.17 µm, average 93.19 × 106.54 µm (n = 50), borne erect or nodding, breaking into two equal pieces. Sporangiophores were hyaline, 11.17 to 34.57 µm wide, average 19.67 µm (n = 50). Columellae were hyaline, pyriform or obovoid to cylindrical, with a distinct basal collar, 32.37 to 102.84 × 23.62 to 68.68 µm, average 60.06 × 40.07 µm (n = 50). Sporangiospores were single celled, mostly ellipsoid, 5.76 to 11.49 × 3.89 to 6.18 µm, average 8.68 × 5.08 µm (n = 100), attaching with 4-5 hyaline appendages at the ends. Chlamydospores were solitary or in short chain, cylindrical or oval. Zygospore was not observed. The isolate was morphologically identified as G. persicaria (Benny 1991). Molecular identification was performed by PCR amplification, sequencing and phylogenetic analysis of the internal transcribed spacer region of rDNA (ITS), partial 28S rDNA large subunit (LSU), and actin-1 (ACT-1) gene using primer pairs ITS1/ITS4, LR0R/LR5 and Gil_ACT_F/Gil_ACT_R (Zhang et al. 2020). The ITS (OP897009), LSU (OR794326), and ACT-1 (OR805109) sequences revealed 99.85%, 99.30% and 100% sequence identity to nucleotide sequences of G. persicaria from NCBI (ON875318, OP243274, and AJ287159). Phylogenetic analysis based on the maximum likelihood method grouped the isolate with other G. persicaria strains. Pathogenicity of the isolate was performed on wounded and non-wounded fruits. Wounded fruits severely rot after 48 h, and no non-wounded fruit rot after 5 days. Therefore, wound was required for the infection of G. persicaria. The pathogen was consistently re-isolated and purified from the inoculated pears, morphologically identified as G. persicaria, fulfilling Koch's postulates. Fruit rot caused by G. persicaria has been reported on peach, tomato, apricot, plum, apple, dragon fruit, papaya and eggplant, as well as Pyrus communis (Mehrotra 1964; Ginting et al. 1996; Cruz-Lachica et al. 2021). This is the first report of G. persicaria infection on 'Housui' pears in China. This disease is a potential threat to 'Housui' pear storage. The confirmation of this soft rot pathogen provides a foundation for pear postharvest disease prevention.