Efficacy of Mutant Strain Derived from Antagonistic Bacteria Isolated from Rhizosphere Soil for the Control of Cucumber Seedling Dampingoff Caused by Pythium aphanidermatum

Abstract

Mutation of antagonistic bacteria isolate KR7 (wild type strain) was induced by using ultraviolet (UV) radiation. Cell suspension of bacteria isolate KR7 was exposed at a distance of 15 cm under the UV lamp for 1−5 minutes before serial diluted and spread onto nutrient glucose agar (NGA). Fourty-seven mutant isolates were obtained from UV-induced mutation and among these isolates, thirty-three isolates were resistant to 100 ppm Rifampicin antibiotic. All these isolates were tested for the efficacy to inhibit mycelial growth of Pythium aphanidermatum (Pa), the causal agent of cucumber seedling damping−off by dual culture technique on potato dextrose agar (PDA). The result showed that nine mutant isolates effectively inhibited mycelial growth of a pathogen by 40.38%−52.38%. These nine mutant isolates were further tested for the disease control efficacy under greenhouse condition. Cucumber seeds were soaked in 10⁸ cfu/ml antagonistic bacterial cell suspension for 15 minutes before sowing in Pa-infested soil and compared with the Pa-inoculated control (+Pa), metalaxyl and Larminar^® (Bacillus subtilis). After sowing for 7 days, nine mutant isolates provided 73.33%−100% of seed germination with were higher than a Pa-inoculated control (+Pa). After sowing for 14 days mutant isolates KRM7.31−Rif and KRM7.13−Rif provided 99.33% and 86.67% of survived seedlings respectively. These survival percentanges were significantly higher than the Pa-inoculated control (+Pa) but were comparable to the wild type isolate KR7. Both isolates also reduced pathogen inoculum in soil, increase percentanges of root colonization and promoted growth of cucumber seedlings as compared with the Pa-inoculated control (+Pa).