First Report of Gilbertella persicaria Causing Soft Rot of Fig Fruit in Japan

Fig (Ficus carica L.), one of the earliest domesticated fruit trees, is widely cultivated in the world due to its ability to adapt to diverse environmental conditions. In August 2024, a rot disease was observed on mature fruit of fig trees cultivated in the Toyo Institute of Food Technology experimental orchard (34°49′08.60″N, 135°24′11.60″E) on sunny days after rainfall with an incidence of 20%. The symptoms appeared as water-soaked lesions, followed by a soft, mushy rot of fruits covered with white to black fluffy mycelia. To determine the causal agent, pieces of diseased fruit tissue were surface disinfected with 70% ethanol, rinsed thrice with sterile water, and placed on potato dextrose agar (PDA) medium. After three days of incubation at 25°C in the dark, the two isolates with similar morphology, SHK1000 (from cultivar San Piero) and SHK1010 (from cultivar Dottato), were obtained by the hyphal-tip isolation under a stereoscope. Each isolate colony was initially white and then turned black, and grew with an average rate of 50.34 mm in diameter per day at 25°C on PDA. Sporangiophores were hyaline to light brown, aseptate, 14.56 to 33.75 µm (average 18.80 µm; n = 50) wide. Sporangia were initially white but became black at maturity, globose, 59.68 to 164.54 µm (average 87.98 µm; n = 50) in diameter, and split longitudinally into two halves. Columellae were hyaline, pyriform to obovoid, 31.06 to 70.66 × 31.14 to 81.28 µm (average 44.56 × 52.38 µm; n = 50), with a basal collar. Sporangiospores were mostly globose to ellipsoid, 4.80 to 11.72 × 3.34 to 9.02 µm (average 7.46 × 5.52 µm; n = 100), with filiform appendages at the ends. Chlamydospores were solitary or in chains, ovoid or globose, light brown to brown, 17.20 to 34.04 × 15.52 to 27.34 µm (average 23.08 × 19.82 µm; n = 50). Zygospores were not observed. The fungal isolates were morphologically identified as Gilbertella persicaria (Benny 1991; Chen et al. 2024). For molecular identification, the internal transcribed spacer of rDNA (ITS), 28S rDNA large subunit (LSU), and actin (ACT-1) were amplified using the primer sets ITS4/ITS5 (White et al. 1990), LR0R/LR5 (Rehner and Samuels 1994; Vilgalys and Hester 1990), and Gil_ACT_F/Gil_ACT_R (Zhang et al. 2020), respectively. The nucleotide sequences of ITS, LSU, and ACT-1 (LC859118–LC859123) showed 100, 99.62, and 100% identity (each 100% coverage) to the sequences of G. persicaria in NCBI (ON875320, MH866729, and AJ287159, respectively). Phylogenetic analysis revealed that the isolates can be grouped into the same clade as other G. persicaria strains from different hosts. To confirm pathogenicity, six unwounded and wounded fruits were sprayed with conidial suspension (1 × 106 conidia/ml) or sterile water as a negative control. Each sprayed fruit was kept in a moist chamber at 24°C. Three days after inoculation, all fruit inoculated with fungal conidia showed symptoms similar to those observed in the field, whereas the control fruit did not show any symptoms. The fungus reisolated from the diseased fruits was confirmed as G. persicaria based on morphological and molecular identifications, fulfilling Koch’s postulates. Fruit rot caused by G. persicaria has been reported on peach (Ginting et al. 1996), strawberry (Zhang et al. 2020), and pear (Chen et al. 2024). To my knowledge, this is the first report of fruit rot on F. carica caused by G. persicaria in Japan. This study provides a basis for developing effective disease management strategies for the pathogenic fungi on figs.