EFFICACY OF FOLIAR APPLICATION OF PHOSPHATES IN CONTROLLING POWDERY MILDEW FUNGUS ON FIELD-GROWN WINEGRAPES - EFFECTS ON CLUSTER YIELD AND PEROXIDASE-ACTIVITY IN BERRIES

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 startin g at 10-cm shoot length on field-grown Chardonnay winegrapes. Both pho sphates and systemic fungicides inhibited development of powdery milde w fungus (Uncinula necator, Schw., Burr.) on fruit clusters, as compar ed with untreated control vines. Disease severity on clusters of plant s 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 a nd 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 tr eated with Dorado and K2HPO4, respectively. Powdery mildew infection r emarkably reduced the weight of non-treated control clusters as compar ed with Dorado and phosphate treated clusters. Phosphate treatment cau sed an increase (3-fold) of peroxidase activity in the soluble fractio n of non-infected control berries. A remarkable peroxidase enhancement was detected in the soluble (S-fold) and ionically bound (2-fold) fra ctions from the phosphate-treated and infected berries. Results indica te that phosphates can be used as foliar fertilizers for disease contr ol in the field and that peroxidase might be involved in the defense p rocess.