First Report of Summer Canker Caused by Pectobacterium carotovorum subsp actinidiae on Kiwifruit in China's Sichuan Province

Kiwifruit (Actinidia spp.) is an economically important fruit worldwide. In August 2016, bacterial canker-like symptoms were observed on the stems of Actinidia chinensis cv. Hort16A in several commercial orchards in Pujiang County. Symptoms were endemic in various townships with sporadic outbreaks, but 3 to 5% of infected plants were observed in three orchards in the town of Daxing. The symptoms were similar to bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa), except that Psa infection normally occurs in the early spring instead of the hot summer. In affected trees, the disease manifested as cankers with exudates on trunks and twigs, and the stems turned brown and slowly died. A total of 39 bacterial colonies were isolated in LB (lysogeny broth) medium from infected shoots. The results of BIOLOG microbial identification showed that the colony phenotypes were uniform, and isolate SCPJ-1 was selected as a representative strain for further tests. The isolate was gram-negative, facultatively anaerobic, rod-shaped, sensitive to tetrazolium, positive to raffinose, and induced a hypersensitive reaction when inoculated in tobacco plants. Molecular identification was performed to identify the species by sequencing a 1,411-bp portion of the 16S rRNA gene; a sequence from the isolate (KY307837) showed 100% identity with the sequence of Pectobacterium carotovorum subsp. actinidiae strain KKH3 (NR125539.1) and 99% identity with the sequence of P. carotovorum subsp. carotovorum (e.g., AB680554.1). PCR amplification with PcaF and PcaR primers (Lee et al. 2015) specific for P. carotovorum subsp. actinidiae produced a 336-bp product that was sequenced (KY315740), which showed 100% identity with P. carotovorum subsp. actinidiae strain KKK3 (FJ969378.1) but 93% identity or less with other subspecies (e.g., CP009678.1). Based on the above results, the isolate was identified as P. carotovorum subsp. actinidiae. For pathogenicity tests, 10 segments of fresh, healthy shoots of A. chinensis from the field were wounded with a 5-mm-diameter stopper borer and inoculated with 20 μl of a 3 to 4 × 106 CFU/ml bacterial suspension, and control shoots were wounded in the same manner and treated with sterile distilled water. Samples were incubated at 33°C in moist chambers, and after 7 days, all treated shoots oozed rust-colored exudates from wounded sites or bud eyes, but the control shoots remained symptomless. Pathogenicity tests were carried out three times under the same conditions, and the pathogen was reisolated and identified from symptomatic shoots by the above-mentioned methods, and control samples could not detect the pathogenic bacteria, thus fulfilling Koch’s postulates. P. carotovorum subsp. actinidiae has previously been reported as a new bacterial pathogen causing kiwifruit canker-like symptoms in Korea (Koh et al. 2012). To our knowledge, this is the first report of P. carotovorum subsp. actinidiae causing kiwifruit bacterial canker-like symptoms in China, and first report of this disease outside of Korea. Identification and monitoring of this new disease-causing agent, which can lead to economic losses, is important and should be considered as a standard procedure in countries where the pathogen is reported. © 2017, American Phytopathological Society. All rights reserved.