Detection of Ralstonia solanacearum in soil and weeds from commercial tomato fields using immunocapture and the polymerase chain reaction

A detection assay for Ralstonia solanacearum in soil and weeds was developed by combining immunocapture and the polymerase chain reaction (IC-PCR). Anti-R. solanacearum polyclonal antibodies were produced in a white female rabbit and Dynal(R) superparamagnetic beads were coated with purified immunoglobulinG (IgG). Using IC-PCR, the 718 bp target DNA was amplified at a detection threshold of approximately 10 4 colony-forming units (CFU) bacteria per millilitre of suspension. DNA was not amplified in soil suspensions derived from autoclaved and non-autoclaved soils, which contained R. solanacearum at 1-10(5) CFU/g soil. However, a positive PCR result was obtained when bacteria in the soil suspensions were first enriched in nutrient broth. IC-PCR detected R. solanacearum in tomato stems 24 h after inoculation by stem puncture with a suspension containing approximately 10 5 CFU/ml. IC-PCR detected the bacterium in 28 of 55 (51%) weeds and 10 of 32 (31%) soil samples. Of the weeds, Physalis minima, Amaranthus spinosus and Euphorbia hirta had the highest incidence of infection. R. solanacearum was not detected in soil taken from fallow fields, but it was discovered in some weed species. Symptomless tomato and pepper plants collected from the fields in which tomato bacterial wilt had previously occurred were found to contain R. solanacearum. These discoveries suggest that weeds and latent hosts may play a role in the survival of R. solanacearum between cropping cycles.