First Report of Pseudomonas viridiflava Causing Bacterial Blight on Globe Artichoke in Turkey

Abstract

The production of globe artichoke (Cynara scolymus L.), which is a major part of Mediterranean cuisine and packs an abundance of nutritional value, has been increasing over recent decades in Turkey. In the spring of 2018, globe artichoke (cv. Sakiz) plants with blight symptoms on outer bracts, dark brown sunken lesion on inner sides, red-brown areas on margins, and small and malformed buds were found in artichoke-growing areas in the Karaburun district of Izmir, Turkey. In some areas, disease incidence was up to 5% of the plants. Several buds in harvest stage exhibiting symptoms were collected from 30-ha artichoke production fields that have heavy soil texture and humid weather conditions and processed to identify the causal agent on 15 artichoke samples. Symptomatic outer bracts showing characteristics of bacterial blight symptoms as well as spines were surface sterilized with 1% NaOCl, rinsed in sterilized distilled water (SDW), and cultured on King’s medium B (KB) for 24 h at 28°C. After incubation, a predominant bacterium was isolated from all samples. Fifteen isolates from independent samples were subjected to biochemical and pathogenicity tests. Round, flat, and blue-green translucent colonies fluorescing under 360-nm UV light when grown on KB medium were observed in all samples taken. The isolates caused soft rot of potato tubers, induced a hypersensitive reaction in tobacco leaves, gave a reaction in the anaerobe respiratory test, and were negative for levan, oxidase, arginine dihydrolase, gelatin hydrolysis, aesculin hydrolysis, and starch hydrolysis. The isolated strains presented the following LOPAT profile: – – + – +. Acid was produced from mannitol, erythritol glycerol, D-sorbitol, D-mannose, and inositol but not from L-tyrosine, D-cellobiose, D-arabinose, and D-trehalose, all of which are typical characteristics of Pseudomonas viridiflava (Lelliott et al. 1966; Schaad et al. 2001). The bacterial identity was further confirmed by amplifying the 16S rRNA gene using the universal bacterial primers 8F and 1492R (Galkiewicz and Kellogg 2008) and gyrase subunit B (gyrB) gene using primers gyrB-Rps/gyrB-Fps (Sawada et al. 1999). The sequences were submitted to GenBank under accession numbers MH763694 for 16S and MK546431 for gyrB. BLASTn analysis of the 1,042-bp amplicon for 16S and 545-bp amplicon for gyrB indicated that it had 99% similarity with P. viridiflava strain 4G1113 (accession no. MG972916) and 100% similarity with P. viridiflava ATCC 13223 strain (accession no. KY612187), respectively. Pathogenicity tests were conducted twice on outer bracts and main stems of artichoke cultivar Sakiz by inoculating 20 µl of bacterial suspension of a 24-h culture in KB medium (1.9 × 10⁹ CFU/ml in SDW). The inoculated plants were placed in a transparent box for 7 days and kept at 25°C and 70% relative humidity. The bract leaves inoculated with only SDW did not develop symptoms; however, the bracts treated with isolated bacterial suspension developed blight symptoms similar to those observed in the field, 7 days postinoculation. The stems inoculated with the isolate also exhibited soft rot symptoms, and the same bacterium was consistently reisolated from both bracts and stems but not from parts of negative control plants. Reisolated bacteria exhibited the same biochemical profile with our isolate, thereby fulfilling Koch’s postulates. P. viridiflava causing a bacterial blight of artichoke bracts was previously reported in Greece (Sarris et al. 2012). To our knowledge, it is the first report of this bacterium causing bacterial blight on leaves and soft rot on stems of globe artichoke in Turkey. Given the availability of climatic conditions, the potential of disease spreading can cause a serious reduction in yield and marketability of this vegetable, which has increasing popularity owing to having high nutritional value, leading to substantial economic losses.