First Report of Fruit Scab Caused by Alternaria alternata on Actinidia chinensis in Korea

Kiwi (Actinidia chinensis) is an economically important fruit in Korea, with 1,300 ha cultivated and a production of approximately 25,000 tons per year (Kim and Choi 2023; Kim and Koh 2018). In late June 2020, fruit scab symptoms were observed on A. chinensis var. rufopulpa in an orchard in Suncheon, Korea. The incidence of scab symptoms among 20-year-old trees was over 75%, primarily superficial, but rendered the fruit less marketable. In the initial stages of the disease, small, light-brown, circular, and oval spots were formed. As the superficial spots expanded, they became cracked scabs measuring 1 to 7 cm with light edges at the later stages. To isolate the causal pathogen, two lesions were cut from two sections of symptomatic tissue, from each of seven fruits from seven trees. Lesions were surface-sterilized with 70% ethanol for 1 min and washed three times with sterilized distilled water (SDW). The sterilized pieces were placed on potato dextrose agar (PDA) and incubated in the dark at 25°C for 1 week. After subculturing on PDA, single-spore isolation produced 14 isolates: SYP-410 to SYP-423. All 14 colonies appeared grayish green and cottony on PDA after 7 days. Conidia were pale brown, ellipsoid to obclavate, with ornamented walls, 1 to 6 transverse and 0 to 3 vertical septa, and length × width of 21.5 to 53.4 × 7.3 to 19.2 μm (average 33.0 × 12.0 μm, n = 100). Their morphological characteristics were consistent with Alternaria spp. (van der Waals et al. 2011; Woudenberg et al. 2015). We randomly selected three isolates from the morphologically similar cultures and named them SYP-412 to SYP-414 for further investigation. The ITS (GenBank accession nos. OR901850 to OR901852), gapdh (OR924309 to OR924311), tef1 (OR924312 to OR924314), rpb2 (OR924315 to OR924317), Alt a1 (OR924318 to OR924320), endoPG (OR924321 to OR924323), and OPA10-2 (OR924324 to OR924326) sequences from SYP-412 to SYP-414 had a 100% (515 bp/515 bp), 100% (578/578), 100% (240/240), 100% (724/724), 95.55% (451/472), 99.33% (445/448), and 100% (634/634) identity with that of the type strain Alternaria alternata CBS 918.96 (AF347032, AY278809, KC584693, KC584435, AY563302, KP124026, and KP124633), respectively. Results from the maximum-likelihood phylogenetic analysis, based on the seven concatenated gene sequences, placed the representative isolates in a clade with A. alternata. Pathogenicity of SYP-412 was tested using 12 surface-sterilized 2-month-old kiwifruits on 20-year-old trees. Six kiwifruits were spray-inoculated with 5 ml of a conidial suspension (1 × 10⁶ conidia/ml) generated after culturing in PDA medium for 7 days, with or without wounding. Another six control fruits were inoculated with SDW with and without wounding. The inoculated kiwifruits were enclosed in plastic bags to maintain high humidity for 1 day. Scab symptoms were observed in both wounded and unwounded fruits 6 weeks after inoculation, but not in the control. The pathogenicity test was performed two times on a total of three separate trees. To satisfy Kochʼs postulates, A. alternata was reisolated from all the symptomatic tissues and confirmed by analyzing the ITS and rpb2 genes. Although scab disease caused by A. tenuissima (now A. alternata) has been previously reported in kiwifruit of A. chinensis var. rufopulpa in China (Ma et al. 2020; Woudenberg et al. 2015), this is the first report of its occurrence on kiwifruit in Korea and will help in future detection and control.