Biotoxic impact of fungicides on plant growth promoting activities of phosphate-solubilizing Klebsiella sp isolated from mustard (Brassica campestris) rhizosphere

The aim of this study was to investigate the side-effects of fungicides on the physiological activities of plant growth promoting rhizobacteria with intrinsic phosphate-solubilizing potential. The fungicide-tolerant and phosphate-solubilizing bacterial strain PS19 was isolated from the mustard rhizosphere and identified as Klebsiella sp. following 16S rDNA sequencing. The Klebsiella sp. strain PS19 normally, produced plant growth promoting (PGP) substances in substantial amount. In this study, four fungicides of different chemical families (tebuconazole, hexaconazole, metalaxyl, and kitazin) at the recommended, two and three times of the recommended rates decreased the PGP attributes of the strain PS19 in fungicide-concentration dependent manner. Moreover, fungicides at the recommended dose had slight inhibitory effect while the dose higher than the recommended ones reduced the PGP traits (phosphate solubilization, salicylic acid, 2,3-dihydroxy benzoic acid, and indole-3-acetic acid production except exo-polysaccharides, hydrogen cyanate and ammonia production) significantly. Of the four fungicides, tebuconazole generally showed the maximum toxicity to the PGP activities of the strain PS19. The results of this study inferred that fungicides, which are used to control various fungal pests detrimental for the crop productivity, must be examined in vitro for their possible adverse impacts on plant-beneficial rhizobacteria before the field application. This study also revealed an additional aspect of the toxicological mechanisms of the fungicides through which they may suppress the plant growth.