Synergistic interactions of biocontrol agents and chemical fungicides enhance the disease resistance in Vigna radiata (L.) against Macrophomina phaseolina-associated with dry root rot

Macrophomina phaseolina, a necrotrophic fungus responsible for dry root rot disease in Vigna radiata L. (greengram) causes extensive yield losses. Biocontrol agents are reported to suppress phytopathogens, mitigate biotic stress, and improve growth and yield of plants. Here, Bacillus subtilis PSB-5 and Trichoderma sp. TD-6 along with chemical fungicides; mancozeb (MZB) and carbendazim (CBZM) were evaluated alone/or in combination for their effectiveness against M. phaseolina-induced root rot disease in greengram. B. subtilis and Trichoderma sp. suppressed phyto-fungal growth by 72 and 78 %, respectively. Both bioagents synthesized indole-3-acetic acid, ammonia, and solubilized insoluble phosphate, released antimicrobial substances including siderophore, hydrogen cyanide, and various extracellular lytic enzymes. Further, efficacy of bioagents and fungicides (at 50 and 100 ppm) were tested (single and combined) against M. phaseolina-infected greengram cultivated in pot soils. While evaluating, bioagents and fungicides increased plant height, biomass, and vigor indices of infected greengram in order: PSB-5 + TD-6 > PSB-5 > TD-6 > MZB + CBZM (100 ppm) > CBZM (100 ppm) > MZB (100 ppm). Further, among treatment, dual inoculation of bioagents (B. subtilis + Trichoderma), maximally increased total chlorophyll (59.4 %), carotenoid (68 %), root phosphorous (69.6 %), root nitrogen (65.7 %), total soluble protein (80 %), soluble sugar (76 %), pod yield (82.3 %) and seed protein (68 %) over infected plants. Moreover, biotic stress-induced oxidative stress biomarkers (proline, malondialdehyde, electrolyte leakage) and reactive oxygen species were significantly (p ≤ 0.05) lowered in fungal-infected greengram following fungicides and bioagents application. Furthermore, combined inoculation of bioagents significantly (p ≤ 0.05) reduced percent disease index (PDI) and increased % protection in biotic-stressed greengram. Besides, bioagents potentially increased disease resistance in M. phaseolina-infected greengram plants by modulating defense responses such as activities of β-1, 3 glucanase, catalase, phenylalanine ammonia lyase, polyphenol oxidase, peroxidase, and superoxide dismutase which significantly protected plants from fungal pathogen. Soil populations of pathogen and bioagents declined and considerably increased, respectively, at harvest. Application of antagonistic bacterial and fungal strains enhances greengram resistance to M. phaseolina while concurrently offer