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 sig nificantly 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 bu pirimate (Nimrod) at 0.5 ml/l, which was applied 4 days before inocula tion with conidial suspension of Sphaerotheca pannosa var. rosae. Dise ase incidence was reduced by 79, 71, 54 and 50 %, as compared to contr ols on plants sprayed with KH2PO4 plus KOH, K2HPO4, NaHCO3 or bupirima te, respectively. Phosphates were suppressive and expressed by disappe arance 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 greenhou se-grown plants were used. Re-application of these salts on the same p lants 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 sh ort-term and not persistent, while the phosphate treatments remained s ignificantly suppressive for up to 23 days, when the experiment was te rminated. The inhibitory and suppressive effectiveness of phosphate sa lts is discussed in the light of their possible acceptance as ideal fo liar fertilizers which should be considered for use in the field for d isease control.