Induced systemic protection to powdery mildew in cucumber by phosphate and potassium fertilizers: Effects of inoculum concentration and post-inoculation treatment

A single spray of 0.1 M solution of phosphate (K2HP04, KH2P04 (pH 4.5 or 9.3) NH4H2P04), or potassium (KC1, KN03, K2S04) salts on the upper surface of the first true leaf of cucumber, before inoculation with Sphaerotheca fuliginea, induced up to 94% systemic protection to powdery mildew on leaves 2-5. Spraying of a 1:1 mixture of both phosphate and potassium fertilizer salts did not improve the protection as compared with that obtained by each of the salts alone. The protection on the upper leaves remained efficient up to 25 days after inoculation regardless of the high concentration of challenge inoculum of S. fuliginea. Postinoculum application of phosphate on the first leaf induced systemic protection against powdery mildew on upper leaves, even when sprayed 4 days after inoculation. The efficiency of induction of systemic protection and curative properties of phosphate and potassium fertilizers can be considered for disease control in the field.