Preliminary tests on in vitro activity of different pure and commercial compounds against Saprolegnia spp.

Introduction: Oomycetes of the genus Saprolegnia are responsible for severe economic losses in freshwater aquaculture. Compounds of proven activity against Saprolegnia spp. (i.e. malachite green) are potentially hazardous to human health and the environment, and their use is forbidden by European regulations. Therefore, the demand for new treatments pushes towards the selection of more safe and environmentally friendly products. In the present work, in vitro activity of two pure compounds (Oxalic acid; Sodium percarbonate) and two commercial products (Actidrox®, De Marco, Italy; Virkon® S, Dupont) was tested on different strains of Saprolegnia. Malachite green was used as reference compound. Methodology: Preliminary trials were performed using two protocols available in the literature by Alderman: one screening method in Agar aimed at assessing the minimum inhibitory concentration (MIC) and one hour bath in aqueous solution of mycelium growing on polycarbonate membrane, to assess the minimum lethal concentration (MLC). Two field strains of Saprolegnia spp. isolated from rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) and one reference strain of S. parasitica (CBS 223.65 furnished by CSICRJB) have been tested in triplicate per each concentration. Results: Our results show that oxalic acid and Virkon® are effective in inhibiting the growth of the mycelium, although at concentrations too high to be applied in the field (MIC 1000 mg/ L). Actidrox® showed a different activity between the two methods (MIC 5000 mg/ L; MLC 500 mg/ L), possibly due to its mechanism of action, that requires presence of water. Tested concentrations of Sodium percarbonate were effective only in slowing down the mycelium growth. Conclusions: Further in vitro trials will be necessary, considering a wider range of promising compounds. The combination of the two methods (inoculation in agar and contact in aqueous solution) represents a good investigation approach for screening the activity of different molecules and products against Saprolegnia spp.