Vine cuttings as possible initial inoculum sources of Ralstonia solanacearum race 1 biovar 4 on vegetable sweet potato in fields

Ralstonia solanacearum, which consists of five races/biovars, is considered a “species-complex” and is an important phytopathogen that causes wilt disease in more than 200 plant species. R. solanacearum race 1 biovar 4 (R1bv4) has caused yield losses of 30–80 % in the vegetable sweet potato (VSP) in the last decade in Taiwan. To identify the source of the initial inoculum of R1bv4 in VSP fields, soil and cuttings from these fields were examined from 2009 to 2010. The results of the investigation indicated that the population of R1bv4 was generally distributed throughout the natural soil of VSP fields at a density ranging from 1.3 × 102 to 9.5 × 105 cfu/g soil; however, the incidence of bacterial wilt was not significantly associated with the density of the R1bv4 population in soils (R2 = 0.084). In contrast, densities of R1bv4 ranging from 2.3 × 103 to 5.9 × 105 cfu/g tissue were detected in the vine tissue of asymptomatic plants in the fields. Additional experiments demonstrated that R1bv4-free VSP cuttings without visible symptoms planted in infested soils in the greenhouse setting could carry approximately 3.1 × 105 R1bv4 cfu/g tissue, which suggests the existence of a latent period for R1bv4 in VSP plants. The results of a BIO-PCR analysis showed that R1bv4 was detected in 2.0 to 98.0 % of the VSP cuttings used for propagation in fields; in addition, the percentage of VSP cuttings carrying R1bv4 and the incidence of bacterial wilt in fields were positively correlated (R2 = 0.909). The inoculation experiments conducted in greenhouses and in fields showed that the cutting inoculum (CI) contributed more to the incidence of bacterial wilt in VSP plants than the soil inoculum (SI). In the field experiments conducted in 2010, an incidence of disease of 27.1 to 38.5 % was detected in healthy field cuttings 8 months after transplantation; in contrast, the incidence of disease in field cuttings carrying R1bv4 was 49.0 to 68.8 %. The incidence of disease was significantly lower in healthy cuttings than in cuttings carrying R1bv4 (p = 0.05).