Evaluation of different salts for the control of lettuce varnish spot: when phytotoxicity rules

Abstract

Five generally recognised as safe (GRAS) salts with antimicrobial activity were investigated for their potential use as bactericides for the control of lettuce varnish spot [Pseudomonas cichorii (Swingle) Stapp]. The phytotoxicity of salts was first assessed using greenhouse and in vitro assays. Greenhouse assays revealed that salts showed different levels of phytotoxicity. Potassium sorbate, sodium benzoate, and sodium carbonate at higher concentrations caused a noticeable decrease of growth along with foliar phytotoxicity symptoms while sodium metabisulfite and sodium bicarbonate caused exclusively foliar symptoms. Based on the phytotoxic doses 5% determined in vitro, salts can be ranked in ascending order of phytotoxicity as follows: sodium bicarbonate, potassium sorbate, sodium carbonate, sodium benzoate, and sodium metabisulfite. When applied at concentrations causing mild to moderate foliar symptoms of phytotoxicity and no noticeable effect on growth, salts did not significantly affect (p ≤ 0.01) survival of P. cichorii on lettuce leaf tissue and did not significantly reduce (p ≤ 0.01) varnish spot severity. Although sodium metabisulfite was applied at concentrations higher than the minimum inhibitory concentration and minimum bactericidal concentration, it did not affect P. cichorii survival on leaf tissue.