Applications of chitosan alone, alternated or combined with copper for grapevine downy mildew management in large scale trials

The persistent use of copper-based fungicides to control grapevine downy mildew (GDM) still represents risks to both human health and the environment, especially in organic production. This heavy metal can affect wine quality due to residues' presence on grape bunches. Implementing sustainable viticultural practices to control diseases is crucial for the wine industry. This work aimed to evaluate chitosan effectiveness toward GDM on a large scale, over a period of three years (2019–2021), and in three distinct vineyards with different environmental conditions. Three GDM management strategies were tested: application of 0.5% chitosan individually; in alternation with copper; and during the last year, in combination with low rate of copper, using conventional strategies and untreated plants as controls. Over three consecutive seasons, chitosan treatments effectively reduced GDM McKinney Index on leaves and bunches under different environmental conditions. On bunches, chitosan alone was less effective than copper, when applied after copper had the same effectiveness, and combined with copper was more effective than copper standard. In 2020 and 2021, the analysis of cooper amount detected in berries of two cultivars showed that all strategies allowed to decrease the amount of copper residue in berries. The application of chitosan alone or in alternation with copper was able to reduce this amount as untreated control. This large scale investigation suggests chitosan as alternative that can integrate and eventually replace copper application against GDM. Further studies to validate these strategies in different environments, disease pressure conditions and cultivar are fundamental towards achieving green and sustainable solutions for GDM management strategies.