A FOLIAR SPRAY OF MICRONUTRIENT SOLUTIONS INDUCES LOCAL AND SYSTEMIC PROTECTION AGAINST POWDERY MILDEW (SPHAEROTHECA-FULIGINIA) IN CUCUMBERPLANTS

A single spray of solutions of 0.005M H3BO3, 0.0025M CuSO4, and 0.0025 MnCl2, on the upper surface of the first true leaf of cucumber plants 2 h before inoculation with a conidial suspension of Sphaerothecafuli ginea, induced systemic protection against powdery mildew in leaves 2 and 3 without causing any damage on the induced leaf (first leaf). A s imilar level of systemic protection was observed when plants were indu ced by micronutrients, 2, 24 and 72 h before challenge with S. fuligin ea. The level of protection induced by various concentrations varied f rom solution to solution. In general, the systemic protection induced by K2HPO4 was similar to that by the microelements. Spraying of a 1:1 mixture of phosphate and micronutrient solutions did not improve the s ystemic protection over that obtained with each of the solutions alone . Increasing the inoculum concentration of S. fuliginea increased the number of powdery mildew colonies produced on both induced and non-ind uced plants and has relatively affected the systemic protection on ind uced plants. A single foliar spray of micronutrient solutions, as a pr ophylactic treatment, on the upper surface of all the leaves of 3-leaf stage cucumber plants significantly inhibited powdery mildew developm ent. A single spray of MnCl2 on leaf 1 elevated peroxidase activity in the soluble fraction and caused an enhancement of beta-1,3-glucanase content in the ionically bound fractions of leaf 2 of non-inoculated p lants. Forty-eight hours after inoculation, the level of both fraction s of the enzymes increased in non-treated plants and decreased (beta-1 ,3-glucanase) or remained unchanged (peroxidase) in treated (induced) plants as compared to non-treated plants. The possible mechanism for t his protection, and the use of microelements and phosphate solutions a s inducers for systemic protection and as agents for disease control a re discussed.