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

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.