NANO-FUNGICIDES: SYNTHESIS AND EFFICACY OF GREEN-SYNTHESIZED SILVER NANOPARTICLES (AGNPS) FOR THE TARGETED MANAGEMENT OF CEREAL SMUT PATHOGENS

Abstract

The cereal crops wheat, barley, and maize are the foundation of global food security, and these are targeted by seed- and flower-infecting smut and bunt pathogens such as Ustilago tritici, Ustilago nuda, Ustilago hordei, Ustilago maydis and several species belonging to the genus Tilletia. These biotrophic fungi are responsible for replacing the normal kernel of grain with masses of dark teliospores resulting in 30–34% yield loss during serious epidemics and reduction of market value as well as seed viability. Synthetic conventional fungicides are effective, but select for resistance, are toxic to non-target organisms and environment, leave residues of chemicals and are financially unsustainable for smallholder farmers. This review is focused on green synthesized AgNPs as a potential sustainable and multitarget nano fungicide for specific management of cereal smut. Botanical extracts and microbial derivatives contain polyphenols, flavonoids, alkaloids, protein, and biosurfactants that act as reducing Ag⁺ to Ag⁰, as well as capping and stabilizing agent during the synthesis process of NPs which are generally spherical and 10–50 nm with surface plasmon resonance between 420 to 450 nm. These green AgNPs exhibit % mycelial growth inhibition from 77 to 100, inhibit teliospore germination and amressorium formation at 12.5-50 μg·mL⁻1 and their multisite mode of action includes cell wall degradation, ergosterol depletion, cell membrane disruption, reactive oxygen species burst, lipid peroxidation, enzymatic inactivation, DNA damage and cAMP signaling suppression, which contribute to limiting the development of resistance. So, seed nanopriming and chitosan-silver nanocomposite coatings enable the targeted delivery, sustained release of ions and also provide additional physiological advantages such as improved germination, activation of antioxidants, and absorption of micronutrients. Selective toxicity to ergosterol-containing fungal membranes rather than phytosterol-containing plant membranes coupled with good ecotoxicological profiles also indicate biosafety. Nevertheless, dose standardization, surveillance of environmental fate, silver accumulation levels, and regulatory issues need to be addressed for commercial use.