Foliar applications of fertilizer salts inhibit powdery mildew on tomato 1 (original) (raw)

Foliar applications of a number of inorganic fertilizer salts were found to significantly reduce powdery mildew [Erysiphe orontii] on greenhouse tomato (Lycopersicon esculentum, cv. Trust) leaves. In a series of singleapplication experiments, the foliar applications, each with 0.1% surfactant, were applied to the third and fourth leaves of young tomato plants 24 h before inoculation with an atomized application of mildew conidia. Control treatments consisted of a water application and a water plus surfactant application. Powdery mildew colonies were counted 7-10 days later. Surfactant alone significantly reduced mildew colony numbers. CaCl 2 , Ca(NO 3 ) 2 , and K 2 HPO 4 reduced colony counts compared with the surfactant alone. All combinations of Ca salts, or Ca salts plus elemental S, significantly reduced mildew colony counts compared with surfactant alone. In a second set of experiments, the effects of repeated applications on already naturally infected tomato plants were evaluated. Young tomato plants were moved to a greenhouse containing several heavily mildew-infected tomato plants, allowing natural infection. The following day and every 7-10 days thereafter, treatments were applied to all leaves. Colony counts on selected leaves were made weekly or just before spraying. Surfactant alone was not as effective as in the single-application treatments, often having no effect. All the Ca-salt treatments that were effective in the single-application series were effective as multiple applications. Repeated applications of combinations of Ca salts were often just as effective as applications of elemental S. KCl, MgSO 4 , and K 2 HPO 4 also significantly reduced mildew counts with multiple applications. This study did not attempt to explain the differences or similarities in efficacy of the salts tested; both osmotic (concentration) and specific-ion effects could play a role.