First Report of Bacterial Canker of Kiwifruit Caused by Pseudomonas syringae pv. actinidiae in Fujian Province, China

In recent years, kiwifruit (Actinidia chinensis cv. Hongyang) was introduced into Fujian Province as an economically important fruit. During 2016 and 2017, symptoms of bacterial canker were observed on kiwifruit in an approximately 15-ha orchard with 3-year-old plants in Fuan city, Fujian Province of China. The orchard exhibited 70 to 80% incidence. Symptoms on leaves began as brown angular spots surrounded by chlorotic margins that later became dark brown spots surrounded by yellow halos, and occasionally red-rusty cankers with reddish exudates were observed on the stems. Two representative strains, FJ-21a and FJ-21d, were isolated onto Luria-Bertani medium (LB) from stems showing a typical symptom of red-rusty cankers with reddish exudates, which were randomly selected from two different plants. These two strains were gram negative, aerobic, rod shaped, and nonfluorescent on King’s B medium or LB; positive for carbohydrate utilization, levan production, and hypersensitive on tobacco and tomato; and negative for oxidase, arginine dihydrolase, urease activity, gelatin liquefaction, and nitrate. The two strains were further identified by polymerase chain reaction (PCR) amplified 16S rDNA region (1,500 bp) and outer membrane protein P1 (ompP1) gene (492 bp) using 27F/1492R (Weisburg et al. 1991) and psaF/psaR (Koh and Nou 2002) primer pairs, respectively. BLASTn analysis showed that the 16S rDNA region (NCBI accession nos. MH071159 and MH071160) and ompP1 (MH084697 and MH084698) gene sequences shared 100 and 100% identity to that of Pseudomonas syringae pv. actinidiae (Psa) strains ICMP 18884 (CP011972) and CRAFRU 12.29 (CP019730), and 11955 (JQ934479) and ISF 8.57 (FR863682), respectively. Based on morphological characteristics, physiological and biochemical tests, and molecular identification, these strains were identified as Psa (Cunty et al. 2015; Takikawa et al. 1989). The two Psa strains were characterized as biovar 3 through PCR using the biovar-specific primers Tac-F/Tac-R (Koh et al. 2014). Pathogenicity was tested with two strains on 6-month-old kiwifruit cv. Jinyan (Fruit Research Institute, Fujian Academy of Agricultural Sciences) using two methods: injecting cell suspensions (10⁸ CFU/ml) into stems with a hypodermic syringe and spraying the bacterial suspension on wounded or unwounded leaves until runoff. Five plants and 10 leaves were used per strain and method. Control plants were inoculated with distilled water or liquid LB medium. The inoculated plants were incubated at 26/22°C (day/night) in a greenhouse (12 h light and >90% relative humidity) for 3 to 4 weeks. As a result, all inoculated plants exhibited the symptoms of twig death or black wilting on stems and yellow or dark brown spots on leaves, which were similar as observed initially in the field. Psa strains were reisolated from each inoculated location. Psa has been reported in other provinces of China, but to our knowledge, this is the first report of Psa causing bacterial canker on kiwifruit in Fujian Province. Psa could have a significant impact on the establishment and productivity of kiwifruit in natural conditions. There is urgency to determine the distribution of Psa in Fujian Province. Furthermore, quarantine measures are needed to prevent further spread of the pathogen to other kiwifruit orchards.