Bacillus subtilis B-2 and selected onion rhizobacteria in onion seedling rhizospheres: Effects on seedling growth and indigenous rhizosphere microflora

Marked strains of Bacillus subtilis B-2 and four selected rhizobacteria were introduced into onion rhizospheres by seed bacterization. Their effects on plant growth and their populations in root surface and root zone soil environments of field-grown onions were estimated 30 days following sowing. Populations of indigenous rhizosphere bacteria and fungi were also assessed by dilution plating. Seed bacterization with B-2 UI-1 and B caused significant increases in shoot height and shoot dry weights of onion seedlings over controls. Only UI-2 caused a significant increase in root dry weight. All of the strains survived in seedling rhizopheres in substantial numbers, but there were marked differences among the strains. Overall, UI-2 and B were comparatively good colonizers, UI-1 and W were intermediate and B-2 was poor. Seed bacterization treatments significantly reduced indigenous bacterial and fungal populations in seedling rhizospheres. Promotion of onion seedling growth by seed bacterization was not correlated with the relative persistance of the introduced bacterium, but may be related to the ability of the introduced bacterium to reduce components of the indigenous rhizosphere microflora.