Post-infection applications of K₃PO₃, phosphorous acid and dimethomorph inhibit development of downy mildew caused by plasmopara viticola on grapevines

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 metataxyl-Cu, were applied at 15-day intervals, starting one week before "veraison" 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 chlorolic 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 phospho-nate and DMM have translaminar activity, and are indicative of uptake by leaf tissue. The inhibitory effectiveness of the phosphorates makes them useful "biocompatible" fungicides for field application for disease control.