Potential of indigenous antagonistic rhizobacteria in the biological control of Monosporascus root rot and vine decline disease of muskmelon

Monosporascus root rot and vine decline (MRVD) of muskmelon, caused by Monosporascus cannonballus, is an economically important disease worldwide. The objectives of this study were to isolate native rhizobacterial strains and to evaluate their ability to promote plant growth and to provide protection against M. cannonballus. Thirty eight native bacterial isolates from the rhizosphere soil of muskmelon and cucumber were screened for their antagonism against M. cannonballus in a dual culture assay. Among them, five isolates viz., Bacillus amyloliquefaciens (B4), Pseudomonas mendocina (B7), Bacillus endophyticus (B10), Pseudomonas resinovorans B11 and P. aeruginosa AT3, identified based on the 16S rRNA gene sequence analysis, showed a significant level of antagonism and the inhibition zone ranged from 5.6 mm to 25.9 mm. Scanning electron microscopic observation of the hyphae of M. cannonballus at the inhibition zone revealed morphological abnormalities including shrinkage, loss of turgidity, pit formation and deformation. These bacterial isolates showed compatibility with one another and with Trichoderma viride. Greenhouse experiments revealed that P. resinovorans B11 was the most effective among the bacterial antagonists in controlling MRVD in melon. When applied as seed treatment and soil application, P. resinovorans B11 reduced the incidence of MRVD by 93.1%, relative to the infected control.