Induction of resistance in grapevines against powdery mildew by Bacillus strains

Abstract

Table grape cultivars are highly susceptible to powdery mildew disease and frequent use of single site action fungicides has led to the development of fungicide resistance in the pathogen and detection of fungicide residues in grapes at harvest. A systematic study was conducted to identify bacteria which can induce resistance in grapevines against the powdery mildew. Two hundred and ninety three bacteria from the grapevine ecosystem were screened in vitro for their growth promoting activities. The twenty two positive isolates, identified as Bacillus species were further screened in vivo for their ability to reduce disease severity and eleven promising isolates which reduced disease severity by 35 per cent or more were identified. These Bacillus species induced higher levels of peroxidase, polyphenol oxidase, total phenols and total proteins in leaves of treated plants and also tested positive for IAA, NH3, HCN and siderophore production, which are implicated in plant growth promotion. The most promising of these, TS-45, DR-92, TL-171, and TP-232 were evaluated, alone or in binary combination, on field grown grapevines for three consecutive seasons during 2015 to 2017. All treatments reduced the powdery mildew severity as compared to the control in all the three trials. Combined applications, of TP-232+TL-171 and TS-45+DR-92 were more effective. Treated vines showed higher level of chitinase, β 1,3-glucanase, peroxidase and polyphenol oxidase activities. In morphological and 16S rDNA gene analysis, isolates TS-45, DR-92, TL-171, and TP-232 showed close homology to Bacillus subtilis, B. endophyticus, B. licheniformis and B. flexus, respectively. This study has shown that soil application of selected efficient Bacillus species can enhance grapevine resistance to powdery mildew disease.