Field evaluation of the bio-efficacy of Bacillus subtilis DR-39 formulation for enhancing pesticide degradation in grapes and optimisation of application dose

In tropical regions of India, a number of diseases and insect pests cause substantial economic damage to the grape crop and timely pesticide applications are required for crop protection. At times, this results in detection of pesticide residue above the specified maximum residue levels (MRL) at harvest affecting international trade. Use of Bacillus species to enhance the degradation of the pesticide residues on grape berries is an emerging option. In this study, the bio-efficacy of a WP formulation of Bacillus subtilis DR-39 in enhancing the dissipation rate of eight pesticides was evaluated during 2016–2017 and 2017–2018 on Thompson Seedless grapes. The pesticides were applied twice as foliar spray at their recommended doses followed by application of B. subtilis DR-39 at 1.0, 2.5 and 5.0 g/l after 4 days of the second pesticide application. In 2017–2018, B. subtilis DR-39 was applied at 2.5 g/l, twice at 4 day intervals. The pesticide residues were analysed on LC–MS/MS by ethyl acetate based extraction method on a time frame and degradation kinetics were calculated. The average enhancement in dissipation by B. subtilis DR-39 was 13.7% @ 1.0 g/l, 19.0% @ 2.5 g/l and 22.7% @ 5.0 g/l. Application of B. subtilis DR-39 @ 2.5 g/l reduced the calculated half-life of the pesticides by 1–3 days, except by 5 and 6.5 days for buprofezin and hexaconazole respectively during 2016–2017, and by 6 days for hexaconazole during 2017–2018. Studies show that B. subtilis DR-39 applications in vineyards can be utilized for faster degradation of multi-class pesticide residues.