الملخص الإنجليزي
Monosporascus root rot and vine decline (MRVD), which is caused by Monosporașcus cannonballus Pollack & Uecker is a destructive disease of muskmelon (Cucumis melo L.) worldwide. Since the pathogen is soil-borne, control of this disease is very difficult. The objectives of this study were to isolate native antagonistic rhizobacteria and to evaluate their ability to promote plant growth and to provide protection against M. cannonballus. A total of 38 bacterial colonies isolated from the rhizosphere soil of muskmelon and cucumber collected from Barka, Al Batinah South Governorate of Sultanate of Oman were screened for their in vitro antagonism against M. cannonballus. Among them, five isolates, designated as B4, B7 and B10, Pseudomonas resinovorans B11 and P. aeruginosa AT3 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 margin of the inhibition zone showed morphological changes including shrinkage, loss of turgidity, pit formation and deformation. On the basis of antagonism, the above five bacterial isolates were chosen for further investigation. Based on the 16S rRNA gene sequencing, the unknown rhizobacterial isolates were identified as Bacillus amyloliquefaciens (B4), Pseudomonas mendocina (B7) and Bacillus endophyticus (B10). Analysis of plant growth-promoting potential of the bacterial antagonists by paper towel method revealed that seed bacteriz bacterial strains had no significant (P<0.05) effect on seed germination percentage, shoot length, root length and seedling vigour. Compatibility analysis of the five bacterial isolates using cross-streak assay revealed no incompatibility among all the tested isolates or with Trichoderma viride. A greenhouse experiment was conducted to evaluate the effectiveness of the bacterial antagonists in reducing the incidence of MRVD of melon plants. Among the biocontrol agents tested, P. resinovorans B11 was the most effective for control of MRVD in melon. When applied as seed treatment and soil application, P. resinovorans B11 significantly reduced the incidence of MRVD by 93.1%, relative to the pathogen control. Application of P. resinovorans B11 as seed treatment plus soil application did not have any significantly negative effects on vine length (P<0.05). Among the bacterial strains tested, P. mendocina B7 alone exhibited endophytic movement in melon plants. To identify the antifungal compound(s) produced by the most promising antagonistic bacterial strain, the cell-free culture filtrate of P. resinovorans B11 was analyzed by thin layer chromatography, which revealed a visible band at 360 nm with an Rf of 0.23. Results of the gas chromatography-mass spectrometry (GC-MS) analysis showed that two compounds with retention time (RT) of 4.25 min and 10.58 min were present in the fluorescent band eluted at Rf0.23. By matching the mass spectra of these compounds with those found in the NIST/ Wiley database, these compounds were identified as 5-hydroxy-2-pentanone (RT: 4.25 min) and hydroxy-4-methyl-2 pentanone (RT: 10.58 min) with molecular weights of 102.13 and 116.16, respectively.
This study is the first proof of the efficacy of P. resinovorans in the management of Monosporascus root rot and vine decline of muskmelon.
