Pig slurry reduces the survival of <i>Ralstonia solanacearum</i> biovar 2 in soil

The effect of added pig slurry and solarization on the survival of Ralstonia solanacearum biovar 2 strain 1609 in soil was analysed in soil microcosms and field plots. In addition, the invasion of potato plants by R. solanacearum and the development of disease symptoms were determined, as measures of induced disease suppressiveness. In untreated soil, R. solanacearum showed slow population declines in both microcosms and the field from, initially, 10(6)-10(7) to 10(3)-10(4) CFU.(g dry soil)(-1) in about 9 weeks. The suppressiveness assays of these untreated soils after this period revealed that most of the plants that were used developed wilting symptoms and (or) contained the pathogen in their lower stem parts, as shown by immunofluorescence colony staining and PCR. The addition of pig slurry resulted in a significantly lower population size of R. solanacearum as well as reduced numbers of infected and (or) diseased plants in the soil suppressiveness tests. On the other hand, solarization of soil also decreased R. solanacearum survival but did not enhance soil suppressiveness as measured by development of disease symptoms and (or) plant invasion after 9 weeks. Combined soil solarization and pig slurry addition showed an additive effect of both treatments. Healthy-looking plants, primarily from soils treated with pig slurry and solarization, incidentally revealed the latent presence of R. solanacearum in the lower stem parts. The mechanism behind the enhanced population declines and disease suppressiveness induced by pig slurry is unclear but shifts in community profiles were clearly discernible by PCR-denaturing gradient gel electrophoresis 9 weeks after pig slurry addition in the field experiment, indicating induced changes in the bacterial community structure.