INDUCTION OF SYSTEMIC RESISTANCE TO POWDERY MILDEW AND GROWTH INCREASE IN CUCUMBER BY PHOSPHATES

A single spray of 100 mM 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 fuligine a, induced systemic protection to powdery mildew in leaves 2 and 3. Ap plication of Na2HPO4, had little or no effect. A mixture of KH2PO4 and Na2HPO4, which was sprayed on leaf 1 however, markedly induced system ic resistance on leaves 2 and 3. Spraying K2HPO4, on leaf 1 at the sam e concentration at 96,48 and 2h before inoculation induced 74,76 and 9 6%, respectively, of systemic protection in the number of powdery mild ew pustules per plant as compared to plants sprayed with water. Consis tently, inductions with K2HPO4, or KH2PO4 were the most effective for systemic protection. One spray of K2HPO4 on leaf 1, two or four days b efore inoculation also stimulated plant growth, regardless of inoculat ion. The possible dual use of phosphate salts as foliar fertilizers an d as agents for induced resistance is considered.