Toxicity Effect of Silver Nanoparticles on Two Plant Growth Promoting Streptomyces Spp. Strains, Phytopathogenic Fungi Fusarium Solani and Phytopathogenic Oomycetes Pythium aphanidermatum and Pythium ultimum

Silver nanoparticles have antimicrobial activity and are used in various commercially produced products. In this study, the effects of two types of nanosilver formulations, including LS2000 and L2000 on two strains of Streptomyces and three phytopathogenic agents, Pythium aphanidermatum, Pythium ultimum and Fusarium solani were investigated. Streptomyces and phytopathogenic agents were cultured on ISP2 and PDA medium respectively supplemented with 0, 5, 10, 25, 50 and 70ppm of LS2000 and L2000. The influence of LS2000 and L2000 on mycelium of Streptomyces was investigated by atomic force microscopy (AFM). Colony forming unit (cfu) of the bacteria decreased in response to elevated concentrations of L2000. LS2000 completely inhibited growth of both strains at a concentration of 5ppm. The inhibitory effects of LS2000 on the phytopathogenic agents were more than L2000. P. aphanidermatum showed the highest tolerance to L2000 and only at 75ppm of the nanoparticles, the diameter of the colonies was decreased. High susceptibility of F. solani to L2000 caused a decrease in fungal colony diameter in lowest concentration of the nanoparticles. The growth of all phytopathogenic agents was decreased by LS2000 and completely stopped in a concentration of 50ppm. The results showed that LS2000 destroyed mycelial networks of the both bacteria in all tested concentrations. Vesicles appeared on the surface of the mycelium branches, subsequent to treatment with L2000. Based on the results, the inhibitory effects of silver nanoparticles on the beneficial soil bacteria were more than on the phytopathogenic agents. Therefore, more caution should be taken in using silver nanoparticles as a fungicide in agriculture.