Two antifungal bacteria were selected from forest soils during the screening of microorganisms antagonistic to Cylindrocarpon destructans, a cause of ginseng root rot. The antifungal bacteria were identified as Bacillus subtilis (I4) and B. amyloliquefaciens (yD16) based on physiological and cultural characteristics, the Biolog program, and 16S rRNA gene sequencing analyses. Antagonistic activity of both bacterial isolates to C. destructans increased with increasing temperature. More rapid starch hydrolytic activity of the bacteria was seen on starch agar at higher temperatures than at lower temperatures, and in the higher density inoculum treatment than in the lower density inoculum treatment. The bacterial isolates failed to colonize ginseng root the root tissues inoculated with the bacteria alone at an inoculum density of 1 × 10 6 cfu/ml, but succeeded in colonizing the root tissues co-inoculated with the bacteria and C. destructans. Scanning electron microscopy showed that the pathogen was damaged by the low-density inoculum treatment with the bacterial isolates as much as by the high-density inoculum treatment. Both bacterial isolates were more effective in reducing root rot when they were treated at a concentration of 1 × 10 6 cfu/ ml than at 1 × 10 8 cfu/ml. Also, only the former treatment induced prominent wound periderm formation, related to structural defense against pathogen infection. The results suggest that the bacterial antagonists may have high potential as biocontrol agents against ginseng root rot at relatively low-inoculum concentrations.
Two similar microbial fungicides (termed as MA and MB) developed in a Korean biopesticide company were analyzed and compared each other in their biocontrol activities against the phytophthora blight of chili pepper caused by Phytophthora capsici. MA and MB contained the microbe Paenibacillus polymyxa and Bacillus subtilis, respectively, with concentrations over those posted on the microbial products. In comparison of the isolated microbes (termed as MAP from MA and MBB from MB) in the antagonistic activities against P. capsici was effective, prominently against zoospore germination, while MBB only significantly inhibited the mycelia growth of the pathogen. Some effectiveness of MAP and MBB was noted in the inhibition of zoosporangium formation and zoospore release from zoosporangia; however, no such large difference between MAP and MBB was noted. In a pot experiment, MA reduced the severity of the phytophthora blight more than MB, suggesting that the disease control efficacy would be more attributable to the inhibition of zoospore germination than mycelia growth of P. capsici. These results also suggest that the similar microbes MA and MB targeting different points in the life cycle of the pathogen differ in the disease control efficacies. Therefore, to develop microbial fungicides it is required to examine the targeting points in the pathogen's life cycle as well as the action mode of antagonistic microorganisms.
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