Phosphites (Phi) are alkali metal salts of phosphorous acid, with the ability to protect plants against different pathogens. In this research, the effect of Phi applied to potato plants on severity of three important potato diseases in Argentina was assessed. Seed tubers and foliage of potato cvs Shepody and Kennebec were treated with Phi to assess effects on resistance against Phytophthora infestans, Fusarium solani and Rhizoctonia solani. Protection resulting from Phi treatment in seed tubers was high against P. infestans, intermediate against F. solani, and low against R. solani. In addition, seed tubers treated with calcium or potassium phosphites (CaPhi and KPhi, respectively) at 1% of commercial product emerged earlier than untreated ones. When Phi were foliarly applied two or four times at different doses, high levels of protection against P. infestans were achieved in both cultivars. Higher protection was observed in Kennebec when CaPhi was applied, while in Shepody this was true for KPhi. Expression of β-1,3-glucanases was induced at different times after treatment but no correlation between β-1,3-glucanases expression and foliar protection level was found. On the other hand, Phi positive protection effects did not produce negative effects in plant growth. Leaves from CaPhi-treated plants showed a darker green colour than leaves from control plants; also an increase in Rubisco protein and a delay in crop senescence was observed.

The protective effect of b-aminobutyric acid (BABA) on two potato cultivars (Bintje and Pampeana) with different levels of horizontal resistance against Phytophthora infestans was investigated during the crop cycle. Plants were treated with BABA 16, 23, 30, 38, 44, 51, 61 or 75 days after emergence. After each application, the percentage of protection and the content of glucanases, chitinases and phenolic compounds were determined in detached leaves. The foliar pretreatment with BABA up to 30 days after emergence showed a 60% protection percentage against P. infestans in cv. Pampeana, while cv. Bintje almost reached 20%. The results indicated that the time of application of BABA and the susceptibility of the cultivars affect both the protection against late blight and the expression of defence molecules like glucanases, chitinases and phenolic compounds.