First Report of Internal Red Spongy Rot of Kiwifruit Caused by Albonectria rigidiuscula in Korea

In December 2025, externally asymptomatic ‘Hayward’ kiwifruit (Actinidia deliciosa) from an export lot originating in Boseong, Jeollanam-do, Republic of Korea, were submitted for diagnosis after internal discoloration was found during cutting at a processing facility in Japan. Affected fruit showed red to reddish-brown, spongy lesions in the flesh, mainly near the stem end. Tissue pieces (5 × 5 mm) from lesion margins were surface-disinfested with 70% ethanol, rinsed with sterile water, air-dried, placed on potato dextrose agar (PDA), and incubated at 25°C for 5 days. Four similar isolates (HWF11, HWF12, HWF13, and HWF16) were obtained from symptomatic fruit; IHWF14 was a representative reisolate from the pathogenicity assay. On PDA, HWF11 produced white to pale pink aerial mycelium with orange to reddish-violet pigmentation and yellow sporodochia after 10 days at 25°C. Microconidia were oval to obovoid, aseptate or 1-septate, produced in chains and false heads on monophialides, and measured 6.3 to 15.0 × 3.3 to 6.0 µm (n = 50). Macroconidia were broad, slightly curved, mostly 5- to 9-septate, with blunt to slightly curved apical cells and foot-shaped basal cells, and measured 56.4 to 99.8 × 5.7 to 9.7 µm (n = 50). Chlamydospores were not observed. These features were consistent with Albonectria rigidiuscula (syn. Fusarium decemcellulare) (Rossman et al. 1999; Zhang et al. 2024). ITS, RPB2, and TEF1 regions were amplified from all five isolates using ITS1/ITS4, fRPB2-5F/fRPB2-7cR, and EF1-728F/EF1-986R, respectively (White et al. 1990; Liu et al. 1999; Carbone and Kohn 1999). Sequences were deposited in GenBank as PZ274982 to PZ274985 and PZ274987 (ITS), PZ310989 to PZ310993 (RPB2), and PZ379638 to PZ379642 (TEF1). BLASTn searches showed 99 to 100% identity with A. rigidiuscula reference sequences. Maximum-likelihood analysis of concatenated ITS, RPB2, and TEF1 sequences in MEGA 12 with 1,000 bootstrap replicates placed all five isolates with A. rigidiuscula reference strains; Nectria cinnabarina CBS 125165 was the outgroup. Pathogenicity of HWF11 was tested on healthy ‘Hayward’ and ‘G3’ fruit. Fruit were surface-disinfested with 70% ethanol, wounded to a depth of 5 mm with a sterile 26-gauge needle (0.45 mm outer diameter), and inoculated with 20 µl of a conidial suspension (1 × 10⁵ conidia/ml); wounded controls received 20 µl of sterile distilled water. Fruit were incubated at 25°C and 90% relative humidity. After 10 days, HWF11-inoculated fruit of both cultivars developed internal red to reddish-brown, spongy lesions, whereas controls showed no internal rot. The assay was repeated three times with three fruit per cultivar and treatment, with consistent results. A. rigidiuscula was consistently reisolated from symptomatic inoculated tissues and confirmed by morphology and ITS, RPB2, and TEF1 sequences, but not recovered from controls, fulfilling Koch’s postulates. Postharvest kiwifruit rots caused by F. solani and F. fujikuroi have been reported (Yang et al. 2018; Pan et al. 2024). F. decemcellulare (= A. rigidiuscula) was reported to cause apple fruit rot in Korea (Lee et al. 2017), but not kiwifruit. Because affected fruit may appear externally healthy, the disease could escape visual sorting and reduce marketability. To our knowledge, this is the first report of internal red spongy rot of kiwifruit caused by A. rigidiuscula in Korea.