Abstract

The abundance and population structure of pseudomonads in soils collected from long-(1006 years)
and short-(54 years) term grapevine monocultures in Switzerland were examined across five soil
horizons within the 1.20e1.35 m range. Soil samples were baited with grapevine, and rhizosphere
pseudomonads containing the biocontrol genes phlD (2,4-diacetylphloroglucinol synthesis) and/or
hcnAB (hydrogen cyanide synthesis) were analyzed by MPN-PCR. The numbers of total, phlDþ and
hcnABþ pseudomonads decreased with depth by 1.5e2 log (short-term monoculture) and 3e3.5 log
(long-term monoculture). In addition, the percentages of phlDþ (except in short-term monoculture)
and hcnABþ pseudomonads were also lower in deeper horizons. RFLP-profiling of phlDþ and hcnABþ
pseudomonads revealed three phlD and twelve hcnAB alleles overall, but the number of alleles for both
decreased in relation to depth. The only phlD allele found in deeper horizons was also found in topsoil,
whereas one hcnAB allele (k) found in deeper horizons in long-term monoculture was absent in the
topsoil. This suggests that certain Pseudomonas ecotypes are adapted to specific depths. Four hcnAB
alleles enabled discrimination between monocultures. We conclude that soil depth is a factor selecting
phlD and hcnAB genotypes, and that the allelic diversity of the two biocontrol genes decreases with
depth.