Induction of Systemic Resistance to Powdery Mildew and Growth Increase in Cucumber by Phosphates

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Abstract
A single spray of 100mM solutions of K2HPO4, KH2PO4, Na4P2O7 and Na3,PO4 on the upper surface of the first true leaf of cucumber plants 2h before inoculation with a conidial suspension of Sphaerotheca fuliginea, induced systemic protection to powdery mildew in leaves 2 and 3. Application of Na2HPO4 had little or no effect. A mixture of KH2PO4 and Na2HPO4 which was sprayed on leaf 1 however, markedly induced systemic resistance on leaves 2 and 3. Spraying K2HPO4 on leaf 1 at the same concentration at 96, 48 and 2h before inoculation induced 74,76 and 96%, respectively, of systemic protection in the number of powdery mildew pustules per plant as compared to plants sprayed with water. Consistently, inductions with K2HPO4 or KH2PO4 were the most effective for systemic protection. One spray of K2HPO4 on leaf 1, two or four days before inoculation also stimulated plant growth, regardless of inoculation. The possible dual use of phosphate salts as foliar fertilizers and as agents for induced resistance is considered.