Seven foliar applications of 0.025M K2HPO4 and KH2PO4 (both plus Tween 20) and the commercial systemic fungicides, Dorado (Pyrifenox) 480 EC, Penconazole EC and Benomyl, were applied at 14-day intervals starting at 10-cm shoot length on field-grown Chardonnay winegrapes. Both phosphates and systemic fungicides inhibited development of powdery mildew fungus (Uncinula necator, Schw., Burr.) on fruit clusters, as compared with untreated control vines. Diseases, everity on clusters of plants treated with K2HPO4 and fungicides was 0.3 and 0.2, respectively, as compared with 1.3 on control clusters (on a 0–4 scale), for the first rating, conducted 10 days after the fifth application of fungicides and phosphates, Five days after the last application, disease severity was 3.5 on non-treated control clusters and 0.3 and 0.8 on clusters treated with Dorado and K2HPO4, respectively. Powdery mildew infection remarkably reduced the weight of non-treated control clusters as compared with Dorado and phosphate treated clusters. Phosphate treatment caused an increase (3-fold) of peroxidase activity in the soluble fraction of non-infected control berries. A remarkable peroxidase enhancement was detected in the soluble (8-fold) and ionically bound (2-fold) fractions from the phosphate-treated and infected berries. Results indicate that phosphates can be used as foliar fertilizers for disease control in the field and that peroxidase might, be involved in the defense process.