The antifungal effectiveness against powdery mildew caused by fungus Erysiphe sp. on the Celastrales plant (Euonymus sp, Celastraceae) by using silver nanoparticles (AgNps) was investigated. Commercial brand (nanonasb pars COM, Iran) of Silver nanoparticles component was prepared. Before use, size particles and stability investigated by spectrophotometer and Scanning Tunneling Microscope (STM). The silver nanoparticles concentration of 4000 ppm was diluted in to 10, 50, 100, 200 ppm and was sprayed on plants surface which were infected by powdery mildew in advance. Under in vitro conditions , cut branch were put in the vials containing mentioned dosages of silver nanoparticles, then antifungal effects were observed by an optical microscope. Under in vivo trails, Plants were investigated for one month after treatment. No remarkable disease reduction was observed, in contrast to the finding of some researchers. Our hypothesis for this contrariety is related to he sensitivity of silver nanoparticles to conditions and materials at field and variety of AgNps at different brands. It is necessary to use standard protocol for these anti microbial and toxicity tests of AgNps and new formulation of AgNps for different crops and conditions.

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