Antimicrobial activity of phosphites against different potato pathogens

Phosphites have low-toxicity on the environment and show high efficacy in controlling oomycete diseases in plants, both by a direct and an indirect mechanism. We have shown that they are also effective in reducing disease symptoms produced by Phytophthora infestans, Fusarium solani and Rhizoctonia solani when applied to potato seed tubers. To gain better insight into the direct mode of action of phosphites on different potato pathogens, and to ascertain chemical determinants in their direct antimicrobial activity, four potato pathogens were assayed with respect to sensitivity toward calcium, potassium and copper phosphites (CaPhi, KPhi and CuPhi, respectively). The influence of acidification and ionic strength changes after Phi addition on the antimicrobial activity, and the fungicidal or fungistatic activity, were evaluated. Results showed that phosphites were able to inhibit growth of all pathogens. Phytophthora infestans was the most inhibited pathogen by all phosphites, followed by Streptomyces scabies, while Rhizoctonia solani and Fusarium solani were less inhibited. CuPhi had the highest antimicrobial activity against the four pathogens analysed, and CaPhi and KPhi showed similar antimicrobial activities. Inhibitions by CuPhi and CaPhi could be partially explained by acidification of the media. However, results obtained with KPhi demonstrated that the phosphite anion has antimicrobial activity itself. The increase in ionic strength after Phi addition was not important in the antimicrobial activity of Phi. The activity of phosphites on germination of F. solani spores showed to be fungistatic rather than fungicidal.