| Summary: | We examined the effects of applications of N, P, K, Mg, and Ca through an irrigation solution and spraying K, Ca, and Mg salts on cucumber powdery mildew (CPM, <i>Podosphaera xanthii</i>) in potted plants and under commercial-like conditions. Spraying CaCl<sub>2</sub> and MgCl<sub>2</sub>, or KCl and K<sub>2</sub>SO<sub>4</sub>, decreased CPM. There were significant negative correlations between the anion-related molar concentrations of the salts and disease severity. Among the sprayed treatments, NaCl provided significantly less CPM control when applied at a low (0.05 M) concentration, as compared with CaCl<sub>2</sub> and MgCl<sub>2</sub>. When sprayed applications of Mg and K salts were analyzed separately from the untreated control, the Cl<sup>−</sup> salts were found to be more effective than the SO<sub>4</sub><sup>−2</sup> salts. High N and Mg concentrations in the irrigation water delivered to young, fruit-less cucumber plants reduced CPM, whereas more CPM was observed when the irrigation solution contained a medium amount of P and a high amount of K. In contrast, mature, fruit-bearing plants had less severe CPM at higher N, lower P, and higher K levels. Spraying mature plants with monopotassium phosphate, polyhalite (K<sub>2</sub>Ca<sub>2</sub>Mg(SO<sub>4</sub>)<sub>4</sub>·2H<sub>2</sub>O), and the salts mentioned above over an entire growing season suppressed CPM. CPM severity was also reduced by spray applications of Ca, Mg, and KSO<sub>4</sub><sup>−2</sup> and Cl<sup>−</sup> salts. Spray applications provided better CPM control than fertigation treatments. Induced resistance is probably involved in the effects of nutrients on CPM.
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