The effect of silver and other metal ions on the <i>in vitro</i> growth of root‐rotting <i>Phytophthora</i> and other fungal species

Abstract

Summary A range of metal ions and the oxoanion WO 4 2‐ were toxic to zoospores of Phytophthora nicotianae parasitica in the order: Ag + &gt; Cu ++ &gt; WO 4 2‐ &gt; Ni + &gt; Co ++ &gt; Zn + . The LD 50 for Ag + , 0.11 μM (11.4 ppb), compared with 1.84 μ m (117 ppb) for Cu ++ . Silver was similarly toxic to a range of pathogens including Pythium aphanidermatum, Thielaviopsis basicola and Fusarium oxy‐sporum f.spp. Most zoospores of Phytophthora spp. were killed by Ag + in the range 46–460 nM (5–50 ppb), bursting at the higher concentrations. A small sub‐population of most propagules exhibited greater tolerance to silver than the whole. In 0.93 μM (100 ppb) Ag + 1.4% of P. nicotianae parasitica zoospores survived but were all killed at 500 ppb. A population of P. cryptogea (1.9%) surviving 0.47 μ m (50 ppb) were killed at 0.93 μM (100 ppb). Zoospore cysts and germlings showed the same sensitivity to silver. Oospores were mostly killed over the range 0.23–0.93 μ m (25–100 ppb) Ag + , some surviving up to the lethal concentration of 9.26 μM (1000 ppb). Mycelium of P. cryptogea was generally less sensitive, with some growth occurring at 9.26 μ m (100 ppb). Zoosporangiogenesis was unaffected over the range 0.47–4.65 μ m (50–500 ppb). Toxicity increased with increased pH over the range 5.0–6.5. Ionic silver was lost from solution during a microscope slide bioassay by binding to the glass surface. In the presence of chloride ions, colloidal AgCl formed which was equally toxic to P. cryptogea . Silver and AgCl were further lost from solution by colloidal agglomeration ‐ Ostwald ripening ‐ and by AgCl adsorption to glass. Silver, &lt; 90 nM (10 ppb) Ag + as AgNO 3 and particles of silver chloride were both strongly attractive to zoospores of P. cryptogea . Spores burst or failed to germinate on entering lethal concentrations. The results are discussed in the context of the use of silver salts to control Phytophthora root‐rot pathogens and the importance of ion availability in in vitro toxicity assays.