Stimulating disease suppression in soils: sulphate fertilizers can increase biodiversity and antibiosis ability of root zone bacteria against Streptomyces scabies

The value of ammonium sulphate (+SO4) fertilizer as a method of controlling potato common scab varies from field to field and from year to year. We examined the influence of two fertilizer regimes on the biodiversity and antibiosis activity (in vitro) of culturable rhizobacterial populations recovered from fields with a similar history of potato common scab. Root zone soils treated with +SO4 fertilizers were colonized by significantly (P=0.05) more species-diverse communities of bacteria than those managed with ammonium nitrate fertilizers (+NO3). In bioassays conducted on artificial media adjusted to pH 4.9 (acid) or pH 6.8 (base), in vitro antibiosis in bacterial strains against Streptomyces scabies was found to be dependent on fertilizer regime (P=0.023), with more isolates with antibiosis ability being recovered from +SO4 compared to +NO3 treated soils. Overall, bacterial species from the genera Bacillus (18–35%) and Pseudomonas (15–18%) were major components of those rhizoflora communities antagonistic to S. scabies. Under our field conditions, soil acidification with +SO4 treatments stimulated the development of rhizobacterial communities that generated secondary metabolites with (in vitro) antibiosis ability, against S. scabies. This occurred both in the liquid and vapour phase (volatile gases); the latter most notably by Bacillus cereus, Bacillus pumilus and Pseudomonas chlororaphis. We postulate that in hot, dry seasons, antibiosis against S. scabies mediated by +SO4 treatments, and the competitive bacterial communities they engender, would be less effective in controlling potato common scab, as the active moieties would be rapidly volatilized into the atmosphere. In contrast, biocontrol would be more effective in wetter seasons, the active moieties being retained, in and around the potato root zone for greater biologically significant periods of time.