THE EFFECT OF SILVER AND OTHER METAL-IONS ON THE IN-VITRO GROWTH OF ROOT-ROTTING PHYTOPHTHORA AND OTHER FUNGAL SPECIES

A range of metal ions and the oxoanion WO42- were toxic to zoospores o f Phytophthora nicotianae parasitica in the order: Ag+ >> Cu++ > WO42- > Ni+ > Co++ > Zn+. The LD50 for Ag+, 0.11 muM (11.4 ppb), compared w ith 1.84 muM (117 ppb) for Cu++. Silver was similarly toxic to a range of pathogens including Pythium aphanidermatum, Thielaviopsis basicola and Fusarium oxysporum f.spp. Most zoospores of Phytophthora spp. wer e killed by Ag+ in the range 46 - 460 nM (5 - 50 ppb), bursting at the higher concentrations. A small sub-population of most propagules exhi bited greater tolerance to silver than the whole. In 0.93 muM (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 muM (50 ppb) were killed at 0.93 muM (100 ppb). Zoospore cysts and ge rmlings showed the same sensitivity to silver. Oospores were mostly ki lled over the range 0.23 - 0.93 muM (25 - 100 ppb) Ag+, some surviving up to the lethal concentration of 9.26 muM (1000 ppb). Mycelium of P. cryptogea was generally less sensitive, with some growth occurring at 9.26 muM (100 ppb). Zoosporangiogenesis was unaffected over the range 0.47 - 4.65 muM (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 pre sence of chloride ions, colloidal AgCl formed which was equally toxic to P. cryptogea. Silver and AgCl were further lost from solution by co lloidal agglomeration - Ostwald ripening - and by AgCl adsorption to g lass. Silver, < 90 nm (10 ppb) Ag+ as AgNO3 and particles of silver ch loride were both strongly attractive to zoospores of P. cryptogea. Spo res burst or failed to germinate on entering lethal concentrations. Th e results are discussed in the context of the use of silver salts to c ontrol Phytophthora root-rot pathogens and the importance of ion avail ability in in vitro toxicity assays.