The ecology of bacterial blight in grapevines caused by Xylophilus ampelinus (Panagopoulos) Willems et al. and the development of effective disease control techniques

The first incidence of bacterial blight caused by Xylophilus ampelinus (Panagopoulos) Willems et al., in grapevines was identified in Hokkaido Prefecture, Japan, in 2009. The aim of the present study was to develop techniques to control bacterial blight by analyzing the habitat and pathogenicity of X. ampelinus, disease susceptibility of grapevine cultivars in Hokkaido, and the effect of germicides. 1. Disease susceptibility Bacterial blight has been diagnosed in regions along the Mediterranean Sea, including Greece, Spain, and France. In these areas, the disease susceptibility of each cultivar has been evaluated; however, these varieties of cultivars are different from those in Hokkaido. Therefore, in the present study, the susceptibility of the cultivars in Hokkaido to bacterial blight was observed using potted grape plants and vines in the field. The results indicated that the following important cultivars were susceptible to the disease: Zweigeltrebe, Seibel 5279, Muller- Thurgau, and Kerner; and cultivars Dornfelder, Cabernet Sauvignon, and Riesling were relatively resistant to the disease. 2. Optimal environmental factors for disease development When disease occurrence was first observed in Hokkaido in 2009, the summer season was predominantly cool and rainy, and it has been suggested that the environmental conditions stimulated disease development. Therefore, the effect of environmental factors on the disease was analyzed. The accumulated time to reach more than 95% relative humidity (RH) and the incidence of diseased leaves were positively correlated. Since other factors were not correlated with disease incidence, the duration of high humidity was considered the most important factor for disease occurrence. 3. Winter habitat To determine the winter habitat of X. ampelinus in Hokkaido, bacterial DNA in the bud, sap, and xylem was examined using polymerase chain reaction (PCR) employing species-specific primer sets. Although X. ampelinus was not detected in the sap and xylem, reported to be the hibernacula in European cultivars, it was detected in the bract and wool of the bud. X. ampelinus in Hokkaido might overwinter in different plant organs than is seen in grapevines in Europe. 4. Genetic analysis Genetic variation was determined in the strains collected from Hokkaido isolated from 2010 to 2013, and Europe isolated from the 1970-80s, using the combined results of Rep-, ERIC-, and Box-PCR. However, the pathogenicity to cultivar Kerner and the virulence were indistinguishable among these strains. 5. Development of a control method The application of a copper agent effectively reduced disease occurrence in Hokkaido, although previous reports indicated that chemical agents did not aid in disease control in Europe. The protective effect of the copper agent increased with prolonged application, and sufficient protection was achieved with 3-4 applications every 10 d when applied concurrent to the initial stage of infection. The application of the copper agent during overwintering was not effective for disease control. 6. Effect of the application of a copper agent on bacterial survival The effect of the copper agent to reduce bacterial overwintering was investigated. The application of the copper agent controlled the extent of infection in the year of treatment, and reduced the quantity of the bacteria surviving winter. Our findings indicate that the regular application of the copper agent would be an effective measure to prevent bacterial blight in grapevines.