Effects of Foliar Sprays of Phosphates on Powdery Mildew (Sphaerotheca pannosa) of Roses

Powdery mildew on plants of a local clone of Rosa indica Major was significantly controlled by a single spray of 25 mM aqueous solutions of K₂HPO₄, KH₂PO₄ plus KOH, or NaHCO₃, all plus Tween 20 (0.5 ml/l) or bupirimate (Nimrod) at 0.5 ml/l, which was applied 4 days before inoculation with conidial suspension of Sphaerotheca pannosa var. rosae. Disease incidence was reduced by 79, 71, 54 and 50%, as compared to controls on plants sprayed with KH₂PO₄ plus KOH, K₂HPO₄, NaHCO₃ or bupirimate, respectively. Phosphates were suppressive and expressed by disappearance of 99% of the pustules and conidia, as early as 2 days after a single spray on mildewed foliage. This treatment was efficient for at least 9 days after the first application when large infected greenhouse-grown plants were used. Re-application of these salts on the same plants reduced the lesion area by about 90% from that recorded before the application. Phosphate and bicarbonate were more suppressive than the systemic fungicide bupirimate in the early period (up to 2 days). The suppresion effects of bicarbonate and bupirimate, however, were short-term and not persistent, while the phosphate treatments remained significantly suppressive for up to 23 days, when the experiment was terminated. The inhibitory and suppressive effectiveness of phosphate salts is discussed in the light of their possible acceptance as ideal foliar fertilizers which should be considered for use in the field for disease control.