Abstract
The use of biocontrol strains recently has become a popular alternative to conventional chemical treatments. A set of bacteria isolated from sugar beet rhizosphere and from roots and shoots of apple and walnut were evaluated for their potential to control sugar beet seedling damping-off caused by R. solani AG-4 and AG2-2.The results of in vitro assays concluded that three isolates, SB6, SB14, SB15, obtained from rhizosphere of sugar beet and five isolates, AP2, AP4, AP6, AP7, AP8, obtained from shoots and roots of apple were the most effective antagonists that inhibited the mycelial growth of both R. solani isolates. Combination of several biochemical tests and partial sequencing of 16S rRNA and gyrBgenes revealed that eight efficient bacterial isolates could be assigned to the genus Bacillus and all could tolerate high temperatures and salt concentrations in their vegetative growth. The potential biocontrol activity of the eight bacterial antagonists were tested in greenhouse condition. The results indicated that four strains,B. amyloliquefaciens SB14, B. pumilus SB6,B. siamensis AP2 and B. siamensisAP8 exerted a significant influence on controlling of seedling damping-off and performed significantly better than others.However, the treatment of the seeds with bacteria was most effective when the isolate SB14 was used, which significantly controlled damping-off disease by 58% caused by R. solani AG-4 and by 52.5% caused by R. solani AG-2-2. This indicates that the use of beneficial bacterial native to the host plant may increase the success rate in screening biocontrols, because these microbes are likely to be better adapted to their host and its associated environmental conditions than are strains isolated from other plant species grown in different environmental conditions. We can infer from the results reported here that sugar beet plantsmay recruitbeneficial microbes to the rhizosphere to help them solve context-specific challenges.
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