The study was performed to evaluate the efficacy of foliar sprays in controlling powdery mildew (Sphaerotheca fuliginea) in greenhouse-grown cucumber (Cucumis sativus L. cv. Delilla) plants. The cucumber plants were grown in a greenhouse in plastic pots containing a mixture of peat, vermiculite and soil (1:1:1, v/v). Twice per week, plants were watered to saturation with a 0,1% 20-20-20 (N-P-K) fertilizer solution. Sprays of KNO3 (20 mM), KH2PO4 + KOH (20 mM) and the fungicide Pyrifenox (Dorado, 0,01% 480 EC, Ciba Geigy, Switzerland) were applied to the upper leaf surface of greenhouse-grown cucumber plants at the five-leaf stage 4 days before inoculation with a conidial suspension of S. fuliginea. These foliar sprays reduced powdery mildew colonies (87%) by 9 days after the inoculation (Figure 1).
Figure 1. Effect of pre-inoculation foliar treatments on control of powdery mildew on cucumber plants. The number of powdery mildew colonies was counted 9 days after inoculation.
In another experiment plants, naturally infected, were transplanted to 10 liter containers. Foliar sprays with 25 mM solutions of KNO3, K2HPO4, KH2PO4 + KOH and the fungicide Pyrifenox were applied at 7 and 14 day intervals, starting seven days after transplanting. Treatments were repeated at 7 and 14 day intervals to give a total of eight and four foliar sprays, respectively. Overall, regardless of 7 or 14 days intervals between applications, KNO3, K2HPO4, KH2PO4 + KOH and Pyrifenox significantly inhibited disease development for all treatments compared to the control (sprayed with water).
The present study clearly demonstrated that simple compounds such as KNO3, K2HPO4 and KH2PO4 + KOH can control powdery mildew on leaves of greenhouse-grown cucumbers as effectively as the systemic fungicide Pyrifenox.