Grapevine downy mildew is one of the major diseases in viticulture. To control this disease, a more effective strategy has been developed and established based on growth and model data as well as on a combination of fungicides. For this purpose, the systemic plant protection product potassium phosphonate (PP) was combined with two contact fungicides. Treatments were carried out according to the different experimental conditions after the growth of 400 cm², 600 cm², and 800 cm² leaf area per primary shoot. PP increased the effectiveness of the preventive fungicides whenever high infection pressure was the case. The experiments also show that it is possible to extend the treatment intervals from 400 cm² to 600 cm² new leaf area when PP was added. However, none of the tested treatments were sufficient for the extension to intervals of 800 cm². These data show that PP can be a key factor in the reduction of the application of synthetic or copper-based fungicides.

Resistance of grapevine against powdery mildew (Erysiphe necator) and downy mildew (Plasmopara viticola) increases during ontogenetic development of leaves and berries. The aim of this study was to determine whether the use of fungicides against both pathogens could be reduced during the last treatment before harvest (LTBH) by treating only the susceptible leaves of the vines in the upper half of the canopy. For this purpose, field trials were carried out under experimental or practical conditions in vineyards in southern Germany in eight different years. All plants were sprayed after a standard regimen until LTBH, performed at BBCH 77–79. LTBH was either performed on the whole canopy or limited to the upper half of the canopy excluding the fruit zone. For both pathogens, no differences were observed between the two treatments with regard to the disease severity on the grape clusters. Only in one out of eight years, an increased fruit disease incidence could be observed. As far as the disease severity of leaves is concerned, no differences were observed for downy mildew, and in the case of powdery mildew, significantly higher infestation was observed only in two out of eight years. Furthermore, the efficacy of different fungicides for LTBH was investigated in a 3-year experiment. Sole use of a copper fungicide for LTBH had no disadvantages against E. necator, compared to a treatment with synthetic fungicides. Taken together, LTBH at BBCH 77–79 can be limited to the upper canopy without risk to the health of the fruit.