First Report of Postharvest Fruit Rot in Longan Caused by Botrytis cinerea in China.

Abstract

Longan (Dimocarpus longan Lour.), a Sapindaceae tropical fruit, is mainly cultivated in China, Thailand, and Vietnam. From 2015-2017, China dominated global production with 1,919.4 thousand metric tons annually (55.7% of total) (Rakariyatham et al., 2020). On Jan 5, 2025, soft rot of longan fruits (10% incidence) was found in the Fruit Wholesale Market of Qujing City (25.54°N, 103.80°E), Yunnan Province, China. The infected fruits exhibited brown epidermis with completely whitened and decayed internal tissues, accompanied by a fermented alcoholic odor. The margins of the infected fruits were cut into small pieces of 0.5 × 0.5 cm, soaked in 75% ethanol for 3 minutes, then rinsed 5 times with sterile water, and placed on potato dextrose agar (PDA), followed by incubation in the dark at 28 °C. After 24h, white mycelia emerged around the tissues, hyphal tip transfer method was used for strain purification on PDA. Four days later, the colony covered the entire 9-cm-diameter PDA and turned gray. Obvious gray sclerotia appeared in the center of the colony after 8 days. Scattered sclerotia appeared around the colony after 14 days. A large number of scattered grayish-black sclerotia were observed after 20 days. Six similar strains were isolated, with a representative isolate YC1126 subjected to morphological and molecular characterization. Morphological analysis revealed septate conidiophores with irregular inter-septal distances, apically branched to form grape-like conidial clusters. The conidia are unicellular, ovoid or ellipsoidal, with a size range of 5.1 to 9.3 μm × 6.5 to 15.4 μm (n = 50). The sclerotia are gray or grayish-black, round or irregular in shape, and the size 0.23 to 1.06 cm × 0.25 to 1.44 cm (n = 50). The morphological characteristics of this isolate are similar to Botrytis cinerea (Liu et al., 2024). Molecular identification was conducted by sequencing the internal transcribed spacer (ITS) region of rDNA and three nuclear protein-coding genes (glyceraldehyde-3-phosphate dehydrogenase [G3PDH], heat-shock protein 60 [HSP60], and DNA-dependent RNA polymerase subunit [RPB2]) using primers of ITS1/ITS4, G3PDHfor/G3PDHrev, HSP60for/HSP60rev, and RPB2for/RPB2rev, respectively (Staats et al., 2005). BLAST analysis showed that this isolate (GenBank accession nos. PQ857502, PV472625, PV472626, and PV472627 for ITS, G3PDH, HSP60, and RPB2, respectively) shared 99.89% to 100% identity with Botrytis cinerea (GenBank accession nos. PV230480 [515/515 bp], MF461628 [922/923 bp], MF461629 [1011/1011 bp], and MN159929 [1133/1133 bp], respectively). A maximum-likelihood and Bayesian posterior probability-based phylogenetic analyses with the concatenated sequences placed YC1126 in the clade of B. cinerea. Thus, isolate YC1126 was identified as B. cinerea. For pathogenicity test, 10 needle-wounded longan fruits were inoculated with 7-day-old fungal discs (with 10 unwounded as control), 10 wounded fruits with conidial suspension (1×10⁶ spores/mL) (with sterile water as control), then incubated at 28°C in the dark. The wounded fruits inoculated with fungal discs and conidial suspension showed the same disease symptoms as previously observed, while the two controls remained disease-free. The pathogen was re-isolated from the symptomatic fruits and was identified as B. cinerea, which fulfilled Koch's postulates. To the best of our knowledge, this is the first report of fruit rot in longan caused by B. cinerea in China.