ANTAGONISTIC EFFECT OF SOME RHIZOBACTERIA ON THE BIOLOGICAL ACTIVITYOF RALESTONIA SOLANISEARUM

Abstract

Bacterial wilt caused by Ralstonia solanacearumphylotypeIIsequevarI(race 3 biovar 2), is considered one of the most destructive bacterial diseases of potato plant.The aim of this study was to evaluate the potential of bacterial antagonists to suppress the causative bacteria. In this regard, 420 isolates ofR.solanacearum were recovered from different habitatsat different Egyptian districts. The variation between isolates was assessed on the basis of pathogenic potentials to tomato seedlings in the greenhouse. The evaluation indicated that, the highest potential, as shown by the wilt severity was obtained by 11 out of 420 isolates. The most virulent R.solanacearum isolates were identified by special techniques. To lay down a biological control protocol, a collection of 318 isolates of rhizobacteria from rhizosphere of different plants, were tested for antibiosis against R.solanacearum selected strains. In vitro, 14 isolates were more effective in suppressing the pathogenat different levels of relative power of antibiosis. These isolates were categorized into three groups according to their Gram-staining reaction, cell morphology and cultural characteristics. Further evaluation, under the greenhouse condition, has shown that six of the isolates were effective in suppressing disease development, as expressed by the Area Under Disease Progress Curve (AUDPC).The assessment of cyanide and siderophore production for selected antagonistis revealed that M3 and M5 isolates gave the highest values of HCN wheaeas maximum siderophore production was recorded by M4, M5 isolates. The selected isolates were identified as: Streptomyces toxytricini,Stenotrophomonasmaltophilia strain IAM 12423 ,Stenotrophomonasmaltophilia strain ATCC 19861 ,Bacillus pseudomycoides andBrevibacillusbrevis. The recovered of antimicrobial compounds from supernatants of these strains were conducted using chlorophorm, petroleum ether and ethylacetat individually to find that chlorophorm phase at 100 ppm for Bacillus pseudomycoides M3 ,Streptomyces toxitricini C5 and Stenotrophomonasmaltophilia M5 gave high broad spectrum of antagonistic effect against selected R.solanacearum strains wheras petroleum ether phase at 50 ppm inhibited the most selected strains of R. solanacearumin vitro.The efficient organic phase of each strain was separated to many fractionated bands by Thin Layer Chromatography(TLC).The antagonistic bacteria promoted the potato growth and inhibited the bacterial pathogens under semi field conditions. The total count of R.solanacearum in the rhizosphere decreased gradually during three months and the count of antagonistic bacteria didn’t change. The growth parameters were improved by the tested rhizobacteria.

Keywords: Microbial antagonists, biological control,B.pseudomycoides,Brevibacillusbrevis,Ralstoniasolanacearum, Streptomyces toxytricini and Stenotrophomonasmaltophilia, PCR,16S rDNA