Foliar spray of botanical extracts influence biochemical processes and plant defence enzymes to ameliorate brown spot induced yield loss in rice

Bipolaris oryzae is the causative agent of rice brown spot disease, leading to serious economic yield losses all over the world. The present study aims to manage brown spot disease on susceptible rice cultivar PR 124 using botanical extracts of three meliaceae species. In vitro screening of botanical extracts revealed the dose dependent response of botanical extracts, though maximum mycelial inhibition occurred at 50% concentration. Ethanolic extracts showed a better inhibition percentage when compared to the aqueous extracts. Azadirachta indica exhibited the highest antifungal ability, followed by Melia azedarach and Toona ciliata. A field experiment conducted during two consecutive years (2021–22) revealed that pre-treatment with promising doses of botanical extracts followed by challenge inoculation 48 h after a foliar spray led to a significant reduction in the severity index, incidence, mean spot count, AUDPC and r value of treated plants over control. A significant increase in yield parameters was also recorded with the foliar application of botanical extracts as compared to un-inoculated (C1) and inoculated (C2) plots. The number of filled grains, test weight, panicle weight and fertility also increased as compared to control plants. The stimulation of the defence system and the antioxidant potential (phenols and lignin) coincided with the induction of endurance in susceptible cultivars. The results demonstrate that defence enzymes like PAL, TAL, glucanase and chitinase are expressed during the activation of a defence response induced by botanical extracts. A. indica was most effective at managing the disease development followed by M. azedarach and T. ciliata. Findings from this research are important for producing a biocide that is safe for the environment and could result in the effective management of this disease.