Concomitant application of AgNPs and four potent Streptomyces spp. against phytopathogen Pythium aphanidermatum exhibited antagonistic activity

Many biological control methods have been evaluated for managing plant root pathogens. One of the most challenging is Pythium aphanidermatum (Edson), a soilborne oomycete causing damping-off, seed rot, and root rot in various crops. In this study, three approaches were investigated: (i) biosynthesised silver nanoparticles (AgNPs) from Aloysia citrodora at five concentrations, (ii) crude extracts of four bioactive Streptomyces isolates (2.5 mg/mL), and (iii) combined applications of Streptomyces crude extracts with 0.01 M AgNPs. Both Streptomyces extracts and AgNPs significantly inhibited pathogen growth compared with controls, with complete inhibition (100%) observed at 0.01 M AgNPs. In contrast, combined treatments exhibited a pronounced antagonistic interaction (P ≤ 0.001), where inhibition ratios declined to 42-55% depending on the isolate. This reduction is likely due to chemical interactions between A. citrodora phytochemicals and Streptomyces metabolites, which may alter nanoparticle stability and diminish their bioactivity. These findings challenge the common assumption of universal synergy between AgNPs and bioactive agents and highlight the need for careful evaluation of interactive effects before developing nano-biocontrol formulations. Importantly, agricultural use of AgNPs remains restricted without regulatory approval, and they should not be applied in natural environments until authorised.