Investigation on Phase Variation Phenomenon in Fluorescent Pseudomonads and their Effect on Control of Gaeumannomyces graminis var. tritici Causal Agent of Take-all Disease

One of the most importanat traits of fluorescent pseudomonads, a phenomenon also related to their biocontrol, is the ability to develop phase variation during their rhizosphere colonization. For an investigation of this phenomenon, at first 24 bacterial strains of Pseudomonas fluorescens that were able to cause an inhibiting zone within the range of 3-14.67 mm. against Gaeumannomyces graminis var. tritici (Ggt) were initially selected for greenhouse experiment. Following the greenhouse experiment, 12 bacterial strains, able to control the take-all disease within a range of %47.5-%77.5 were chosen for studying the phase variation phenomenon in laboratory conditions. The 12 bacterial strains were cultured on King B and SA media on seven of which, viz, Um141, Um11, Um138, Um70, Um115, UmCHN5 and Pseudomonas fluorescens F113 colonies of various morphologies were observed. Results of phase variation phenomenon during rhizosphere colonization in these bacterial strains (both in sterilized conditions and in the presence of Ggt), showed that phase variation had occurred in these strains during the process of rhizosphere colonization and these strains had been able to adopt themselves to root in various growing conditions and as well to penetrate to the end of the root. A comparison of bacterial population in different parts of the root indicated that the most population prevailed at the bottom of the root area. The results of the phase variation phenomenon in the presence of Ggt proved the fact that this phenomenon can lead to a reduction of the rate of take-all disease.