Powdery mildew, caused by Sphaerotheca fuliginea, was significantly controlled by a single spray of aqueous solutions (25 mm) containing various phosphates and potassium salts. Phosphates were suppressive when applied alone; however, treatments in combination with Tween‐20 were more effective in causing the disappearance of powdery mildew pustules from diseased foliage. Efficiency of control, as expressed by the disappearance of 99% of pustules, was recorded 1 or 2 days after application of single sprays of phosphate and potassium salt solutions. Treatment was effective for up to 12 or 15 days, respectively, following application to small or large greenhouse‐grown plants with established mildew infection. Treatments also markedly reduced (> 99%) the production of eonidia from colonies. A further application of these salts to the same plants resulted in the elimination of about 50% of mildew colonies present prior to the application. Further spray application inhibited disease development compared with water‐sprayed plants, but did not reduce the number of existing lesions. Phosphate was more effective than the systemic fungicide pyrifenox and reduced established powdery mildew infection up to 11 days after application, but the converse was true when assessments were made after 15 days. These properties of phosphates and potassium salts make them appropriate for use as foliar fertilizers with a potential beneficial influence on disease control.
A single foliar spray of 0-1 M phosphate salts solution was applied to the upper side of maize (cv. Jubilee) leaves 1,2 and 3 at the five-to six-leaf stage, 2-4 h before inoculation with Puccinia sorghi. This treatment induced systemic resistance against common rust as expressed by 98% reduction in the number of pustules of P. sorghi developed on leaves 5, 6 and 7, There was no damage or chlorotic stippling on the induced leaves (I, 2 and 3) as a result of the phosphate spray, A less effective, but still significant. reduction (90%) in the number of pustules was obtained when the foliar spray was applied 6 days before inoculation. One foliar spray of K2HPO4 on leaves 1,2 and 3.6 days before inoculation, stimulated plant growth regardless of inoculation. The foliar spray on the day of inoculation was less effective in stimulating growth. Appropriate mixtures of phosphate solutions revealed that the level of protection was not necessarily dependent on the gradual changes in the pH of the solution. The possible dual use of phosphate salts as foliar fertilizers and as agents for induced resistance is discussed.
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