Abstract
Silver nanoparticles (AgNPs) were green synthesized using leaf extract of the leaf blight disease (Rhizoctonia solani) susceptible red amaranthus (Amaranthus tricolor L.) and the disease-resistant green (A. dubius) and the wild amaranthus (A. viridis) genotypes, physically characterized, and assessed for their anti-fungal effects against R. solani. The green synthesized nanostructures showed an absorption maximum typical of silver nanoparticles in spectroscopy, and face-centered cubic structures in X-ray diffraction. Field Emission Scanning Electron Microscopic analysis and High-Resolution Transmission Electron Microscopy revealed the size range to be 35-45 nm for all the samples. In vitro mycelial growth inhibition of the pathogen occurred with 500 and 750 ppm concentrations of the nanoparticles in a poisoned-food assay. Further, detached leaves of red amaranthus variety were sprayed with the nanoparticles, and then challenged with the pathogen. There was significant difference in lesion development on leaves sprayed with Ad-AgNPs and Av-AgNPs compared to those treated with At-AgNPs. In the in vivo assay, challenging with the pathogen after spraying the foliage of the leaf blight susceptible red amaranthus variety with Ad-AgNPs at 750 ppm concentration recorded the lowest disease index (7.40) followed by that received Av-AgNPs spray at the same concentration (17.69), five days after inoculation. At-AgNPs treated plants had a disease index of 49.38. Our findings suggest that application of AgNPs green synthesized with leaf extract of disease-resistant genotypes of amaranthus effectively suppressed leaf blight disease incidence in a susceptible amaranthus genotype. To our knowledge, this is the first report on the improved plant pathogen-suppressive activity of any metal nanoparticle when biogenically synthesized using extracts from a disease-resistant plant genotype.