Control of Downy Mildew of Grapevine with Potassium Phosphonate: Effectivity and Phosphonate Residues in Wine

Abstract

ABSTRACT In organic viticulture, downy mildew is often controlled with copper. Because copper is known to be phytotoxic and to accumulate in the soil, the replacement of copper is a major concern of organic agriculture. Here, 13 on-farm trials to evaluate potassium phosphonate as an alternative fungicide to copper are reported. Residues were determined in 53 samples of wine. Potassium phosphonate was effective against downy mildew, but not against powdery mildew and red fire disease. Treatment effectivity against downy mildew increased significantly with the amounts of potassium phosphonate applied. Phosphonate residues in wine were highly correlated with the total amount of potassium phosphonate applied during the vegetation period, but not with the date of the last application. Under equal treatment of the grapes with potassium phosphonate, red wines contained approximately 25% more phosphonate than white wines, but this was not sufficient to result in detectable differences between red and white wines in a random sample of wines. There were no obvious differences among grape cultivars. In conclusion, potassium phosphonate is an effective fungicide for the control of downy mildew, but the application of potassium phosphonate inevitably leads to phosphonate residues in the wine. In our view, phosphonate residues are not compatible with the reputation of organic wine among consumers, and we do recommend the use of potassium phosphonate in organic viticulture.