SUPPRESSION OF CUCUMBER POWDERY MILDEW (SPHAEROTHECA-FULIGINEA) BY FOLIAR SPRAYS OF PHOSPHATE AND POTASSIUM-SALTS

Powdery mildew, caused by Sphaerotheca fuliginea, was significantly co ntrolled by a single spray of aqueous solutions (25 mM) containing var ious phosphates and potassium salts. Phosphates were suppressive when applied alone; however, treatments in combination with Tween-20 were m ore effective in causing the disappearance of powdery mildew pustules from diseased foliage. Efficiency of control, as expressed by the disa ppearance of 99% of pustules, was recorded 1 or 2 days after applicati on of single sprays of phosphate and potassium salt solutions. Treatme nt was effective for up to 12 or 15 days, respectively, following appl ication to small or large greenhouse-grown plants with established mil dew infection. Treatments also markedly reduced (> 99%) the production of conidia from colonies. A further application of these salts to the same plants resulted in the elimination of about 50% of mildew coloni es present prior to the application. Further spray application inhibit ed disease development compared with water-sprayed plants, but did not reduce the number of existing lesions. Phosphate was more effective t han the systemic fungicide pyrifenox and reduced established powdery m ildew infection up to 11 days after application, but the converse was true when assessments were made after 15 days. These properties of pho sphates and potassium salts make them appropriate for use as foliar fe rtilizers with a potential beneficial influence on disease control.