Black Rot of Kiwifruit Caused by Alternaria alternata in Korea

Abstract

Black rot affecting fruits of kiwifruit, ‘Skinny Green’ (Actinidiahybrid) caused by Alternaria were found in a farmer's orchardlocated in Sacheon, Korea, in 2010. The disease was found exclu-sively on cultivar Skinny Green, which was recently developed inKorea. The typical symptoms of the disease wereblack spots ar oundthe remnants of the calyx of the kiwifruit, the spots graduallyexpanded and covered most of the calyx (Fig. 1A). The black fruitrot extended to the inside of fruits, while the outside of the lesionbecome hardened and mummified (Fig. 1B). Infected fruits are nolonger marketable. Alternaria often infects the blossom and young fruit throughwounds and forms abundant conidia on the surface of the lesions.Since the cultivar Skinny Green is often cultivated in vinyl houses,hot and humid conditions during the summer favor disease develop-ment and spread. The infection rate of the disease in the surveyedarea reached 18% in June to July, 2010. Fresh fruits with symptomswere collected from the fields and the causal pathogen was isolatedfrom the calyx of fruits. Colonies formed on potato dextrose agar(PDA) were velvety, olivaceous, black to sooty black, with abundantsporulation and little or no sporulating aerial mycelium, and nopigment release into the medium (Fig. 1C). The optimum temper-ature for growth of the fungus was 25°C Conidia wer. e pale brown togolden brown in color, commonly in moderately long or long chainsof five to ten or more, abundantly or sparsely branched, initiallylong-ellipsoid, becoming ellipsoid to ovoid, obclavate to obpyri-form, mostly beakless (erostrate) or with a narrowly tapered beak.They became conical, and generated secondary conidiophores of13−35×8−15 µm in size Th. ree to five transverse septa were observed,with one to two longitudinal or oblique septa in one or two of thetransverse divisions; a dark median septum was absent (Fig. 1D).Conidiophores were pale to mid-brown in color, smooth, septate,slightly swollen, and hyaline at the apex, solitary or in small fasciles,commonly branched, straight or flexuous, 80 µm long×4 µm inwide. The detailed mycological characteristics ofth e species and thehost range have been reported by Yu (2001). For pathogenicitytesting, a representative isolate of the pathogen was inoculated intofresh kiwifruits through a puncture wound.T he causal pathogen wasre-isolated from the lesions to confirm Koch's postulation (Fig. 1E).For further identification, we amplified and sequenced thecomplete internal transcribed spacer (ITS) rDNA of theisolate usingthe primers ITS1 and ITS4, as described by White et al. (1990). Theresulting sequence of57 0-bp was deposited in GenBank (AccessionNo HQ. 257255) P. hyol genetic analysis was conducted using MEGA4software, with the neighbor-joining method and the Tajima-Neidistance model. Comparison with ITS rDNA sequences showed100% similarity with sequences of Alternaria alternata (GQ249171)(Fig. 2). On the basis of symptoms, mycological characteristics,pathogenicity, and the ITS sequence, this fungus was identified as A.alternata (Fries: Fries) Keissler. Only leaf spot diseases of kiwifruitcaused by Alternaria sp. have been described in Korea and black rot,on kiwifruits caused by A. alternata has not previously beenreported (Jeong et al., 2008). The representative culture of thefungus is stored at National Academy of Agricultural Science(KACC 45440), Suwon, Korea.