Powdery mildews (PM) caused by Sphaerotheca pannosa and Uncinula necator are major foliar pathogens of field-grown nectarine trees and grapevines, respectively. Field experiments using nectarine trees and grapevines were conducted in commercial orchards and vineyards during 1993-2000 in the Golan and the Galilee regions of Israel. Foliar sprays of 1% mono-potassium phosphate fertilizer PeaK® (MKP=KH2PO4), demethylation inhibitors (DMI) fungicides, and an alternating treatment with Peak and DMI inhibited development of PM on fruit and leaves and fruit clusters of field-grown nectarine trees and grapevines, respectively. Alternating the two treatments causes similar results to DMI fungicides in controlling PM. However, application of the systemic fungicides only, omitting the PeaK treatments when they were scheduled, was significantly less effective than either the fertilizer or the alternation treatments. This indicates that the use of PeaK has a significant role in disease control and it can reduce the number of fungicide treatments against PM by at least 50%. Tank-mix treatments of PeaK solution with DMI at a recommended rate provided the best protection against PM. These results were confirmed in large-scale demonstration trials conducted in commercial orchards and vineyards. PeaK solutions were not phytotoxic to plant tissues and did not affect (i) the size of berries; or (ii) pH, total acids, or Brix of the juice, as compared with commercial standards. Fruit harvested from PeaK-treated nectarine trees by either alternation or tank-mix were similar or showed larger size than those harvested from the commercial fungicide-based treatment. Fruit harvested from control, non-treated trees were much smaller. The inhibitory effectiveness of PeaK makes it a potential major component of an integrated disease control program.