Abstract
Two foliar sprays of aqueous solutions of 0.6% K3PO3, 0.3% H3PO3, 0.3% dimethomorph (DMM), and the commercial systemic fungicides metalaxyl-mancozeb and metalaxyl-Cu, were applied at 15-day intervals, starting one week before “verai-son” on field-grown ‘Cabernet Sauvignon’ winegrape. Phosphonates (K3PO3 and H3PO3) and systemic fungicides inhibited development of downy mildew fungus (Plasmopara viticola). While disease severity gradually increased on leaves of control vines from 1.26 to 1.80 (on a 0-4 scale), it was reduced to 0.08, 0.02 and 0.0 on leaves of vines treated with H3PO3, K3PO3, and fungicides, respectively. These treatments prevented sporulation of the colonizing fungus in leaves bearing chlorotic lesions. Severe downy mildew on leaves influenced the juice quality of berries so that the lowest Brix in juice occurred in berries of control vines. An application of 50 ppm aqueous solution of K3PO3 or DMM to leaf disks completely inhibited disease development. These data show that both phosphonate and DMM have translaminar activity, and are indicative of uptake by leaf tissue. The inhibitory effectiveness of the phosphonates makes them useful “biocompatible” fungicides for field application for disease control.