Controlling fusarium wilt disease of cucumber plants via antagonistic microorganisms in free and immobilized states

Abstract

The causative agent of cucumber wilt was isolated from a diseased cucumber plant, grown under green house conditions and identified as Fusarium oxysporum. Forty isolates of exospore-forming actinomycetes and endospore-forming bacteria (twenty isolates each) were randomly isolated from the rhizosphere soil of a healthy cucumber plant. Among these isolates, 8 actinomycetes and 6 spore-forming bacterial isolates exhibited antagonistic activities against Fusarium oxysporum. One isolate of actinomycetes and another one of endospore-forming bacteria, which showed the highest antagonistic activities against the pathogenic fungus were selected and identified as Streptomyces spp. and Bacillus mycoides, respectively. Inoculation of cucumber plants, grown in Fusarium-infected soils with any of the antagonistic microorganisms ( Streptomyces spp. or B. mycoides), resulted in a marked reduction in total count of fungi in the rhizosphere soils and much lower percentages of diseased plants as compared with the uninoculated ones. Higher antifungal activities were achieved by application of these microorganisms in immobilized form (encapsulated in sodium alginate beads) than in case of using the free cells. With application of mixed inocula of these two antagonistic microorganisms in free or immobilized state, lower anti-fungal activities were observed, than in case of using each one separately. Such observation was attributed to the detected antagonistic activity of Streptomyces spp. against B. mycoides. The lowest percentage of diseased cucumber plants was achieved when immobilized Streptomyces spp. was applied as biocontrolling agent to plants grown in Fusarium-infected soils.