Outbreak of Bacterial Blossom Blight Caused by Pseudomonas viridiflava on Actinidia chinensis Kiwifruit Plants in Italy

Abstract

During the springs from 2006 to 2008, a new disease was observed on 4- to 5-year-old Actinidia chinensis (cv. Jin Tao) trees in different commercial kiwifruit-production areas in northern Italy (Lombardy). Initially, disease occurrence was sporadic but later became widespread. Symptoms on flowers appeared as a dark brown rot of anthers, filaments, sepals, and whole buds. Blossoms abscised prematurely from buds. Symptoms appeared as confluent brown spots often present on rolled margins. Bacteria were isolated from symptomatic tissue on nutrient agar medium supplemented with 5% sucrose. The isolated bacteria were aerobic; produced a diffusible fluorescent pigment on King's B medium; levan, oxidase, and arginine dihydrydrolase negative, and catalase positive; rotted potato tuber tissue; caused a hypersensitive response on tobacco; and failed to reduce nitrate or utilize sucrose and were ice nuclease-positive at –5°C, suggesting the organism was P. viridiflava (1,3). Inoculation of 2-year-old A. chinensis cv. Jin Tao plants were carried out in the greenhouse under controlled environmental conditions (15 to 27°C, night/day; relative humidity up to 70%) by spraying five plants in bloom with a suspension (1 × 108 CFU/ml) of isolated bacteria with a hand-held sprayer that produced large spray droplets. Symptoms, similar to those in nature, were observed on flowers and buds 3 to 5 days after inoculation and on leaves after 7 to 10 days. Using the same tests described above with the original strains, the strains that were isolated from symptomatic tissue were identified as Pseudomonas viridiflava. Seven bacterial strains (PV508–PV1108) were identified by sequencing 1,481 bp of their 16S rDNA region (2) and using BlastN (4) for the most similar sequences in the INSD (GenBank, EMBL, and DDBJ). Our sequences shared 99.53% (1,474 of 1,481 bp) to 99.9% (1480 of 1,481 bp) identity with the analogous sequences of P. viridiflava available in the database. To our knowledge, this is the first report of an outbreak of blossom blight caused by P. viridiflava on A. chinensis cv. Jin Tao kiwifruit plants in Italy. Previously, it was reported on A. deliciosa cv. Hayward (3). Because of the risk of bacterial contamination among the different cultivars of kiwifruit, further investigation and development of control measures are in progress. References: (1) R. A. Lelliott and D. E. Stead. Methods for the Diagnosis of Bacterial Diseases of Plants. Blackwell Scientific, Oxford, UK, 1987. (2) E. R. B. Moore et al. Syst. Appl. Microbiol. 19:478, 1996. (3) L. Varvaro et al. Inf. Fitopatol. 6:49, 1990. (4) Z. Zhang et al. J. Comput. Biol. 7(1-2):203, 2000.