Survival of Ralstonia solanacearum biovar 2, the causative agent of potatobrown rot, in field and microcosm soils in temperate climates

After outbreaks of potato brown rot in three different fields in the Nether lands, the fate of the brown rot pathogen, Ralstonia solanacearum biovar 2, was monitored in soil by immunofluorescence colony staining (IFC) supporte d by R. solanacearum division-2 specific polymerase chain reaction. In sele cted areas of all fields, the R. solanacearum population densities were ini tially on the order 10(4) to 10(6) per g of topsoil. These population densi ties then declined progressively over time. In two fields, however, the pat hogen persisted for periods of 10 to 12 months. The survival of a selected R. solanancearum biovar 2 isolate, strain 1609, in three soils, a loamy san d and two different silt loam soils, was further studied in soil microcosm experiments. The effects of temperature and soil moisture content were asse ssed. At 12 or 15 and 20 degreesC, a gradual decline of the population dens ities was observed in all three soils, from the established 10(5) to 10(6) CFU g(-1) of dry soil to significantly reduced levels, occasionally borderi ng the limit of detection (10(2) CFU g(-1) of dry soil), in periods of appr oximately 90 to 210 days. Soil type affected the rate of population decline at 20 degreesC, with the greatest decline occurring in loamy sand soil. In all three soils, the survival of IFC-detectable R. solanacearum 1609 cells at 4 degreesC was severely impaired, reflected in an accelerated decline o f CFU counts, to undetectable numbers. Moreover, indications were found for the occurrence of viable but nonculturable strain 1609 cells in the loamy sand as well as in one silt loam soil under these conditions. In addition, a single freezing-thawing cycle caused a significant additional reduction o f the culturable R. solanacearum 1609 populations in the three soils, thoug h detectable populations remained. Moderate soil moisture fluctuations of a pproximately pF 2 did not affect the survival of R. solanacearumm 1609 in s oil. Severe drought, however, drastically reduced the populations of strain 1609 CFU in all three soils.