Outbreak of Ralstonia solanacearum Biovar 2 Causing Brown Rot on Potato in the Aegean Region of Turkey.

Ralstonia solanacearum (Smith) Yabuuchi, Kosako, Yano, Hotta, and Nishiuchi, the cause of brown rot of potato (Solanum tuberosum), was detected for the first time in Turkey in 1995 in five potato fields in the Nevsehir Province of the central Anatolia Region and was eradicated under measures mandated by the government. Occurrence of the pathogen was not reported in other parts of the country. However, in 2006, brown rot symptoms were observed in potato (cv. Marabel) fields in the Balikesir Province of the Aegean Region. Symptoms and signs included wilting, browning of stem vascular tissues, and ooze exudation from the transversely cut stem. On tubers, brown discoloration of the vascular ring was observed. Creamy bacterial ooze emerged from the vascular ring a few minutes after cutting. In advanced stages, bacterial slime oozed from the tuber heel end (stolon) and "eyes" causing soil particles to adhere. Isolation of bacteria from diseased stem and tuber tissues on mSMSA medium (1) consistently resulted in white, fluid colonies with red coloration in the center. On the basis of biochemical, immunofluorescence (IF), and real-time PCR tests, 10 representative isolates (one per affected field) were identified as Ralstonia solanacearum. They were further identified as biovar 2 according to metabolization of maltose, lactose, and D (+) cellobiose but not mannitol, sorbitol, and dulcitol. In the IF tests, fluorescent cells were observed at antibody dilutions from 200 to 12,800. The expected real-time PCR products were generated using biovar 2-specific primers (2). Pathogenicity tests were performed by injecting a bacterial suspension (10⁶ CFU/ml) into the stem of 2-week-old tomato seedlings (cv. Alta F1). Inoculated plants (five plants per isolate) were incubated for up to 2 weeks at 25°C and 70 to 80% humidity. Wilting symptoms developed within 5 to 10 days. No symptoms were observed on controls inoculated with sterile water. The bacterium was reisolated and identified as R. solanacearum biovar 2 as described above. The incidence of the disease in the affected fields varied between 20 and 40%, and surveys showed that approximately 163 ha were infested. Phytosanitary measures that were taken included a prohibition of production of host plants in the infested areas, tracing and testing programs to identify the source of the bacterium, and measures to prevent any further spread of the bacterium to new areas. To our knowledge, this is the first report of R. solanacearum biovar 2 on potato in the Aegean Region of Turkey. References: (1) J. G. Elphinstone et al. EPPO Bull. 26:663, 1996. (2) M. Ozakman and N. W.Schaad. Can. J. Plant Pathol. 25:232, 2003.