Soil conduciveness to take-all of wheat: Influence of the nitrogen fertilizers on the structure of populations of fluorescent pseudomonads

Abstract

In a field cropped with wheat, a high and low level of soil conduciveness to take-all were induced by applying a nitrogen fertilizer with either calcium nitrate or ammonium sulphate. From these two soils, two representative populations of fluorescent pseudomonads were tested for their in situ behaviour. Take-all index and root dry weight were assessed on plants cropped in soils infested with Gaeumannomyces graminis var tritici (Ggt) and each bacterized with one of the isolates of fluorescent pseudomonads. The bacteria tested can be split into three groups: antagonists which reduce take-all, deleterious isolates which aggravate the disease and neutral without evident effect on the disease. The predominance of antagonistic fluorescent pseudomonads in the NH4-treated soil and the predominance of deleterious ones in the NO3-treated soil was confirmed after statistical analysis. The microbial impact on take-all must be more considered as the resulting effect of divergent activities of both rhizobacteria types than the only consequences of the presence of antagonistic pseudomonads. All the high cyanogenic pseudomonads were antagonists in situ and were more numerous in the NH4-treated soil than in the NO3-treated soil.