Chitosan treatment to manage grapevine downy mildew

Abstract

Context and purpose of the study

Downy mildew is one of the most important grapevine diseases, caused by the Oomycete Plasmopara viticola. The management of the disease in organic agriculture can require up to 15 copper applications per year. However, copper accumulates in the soil, is phytotoxic and is toxic for organisms living in the soil, its use has been restricted in European Union to maximum 28 kg in 7 years. Therefore, testing of alternatives with equal effectiveness is desirable. Among those, the natural biopolymer chitosan, obtained from crab shells, proved to be effective toward downy mildew in plot experiments. The aim of our trials was to extend chitosan applications in large scale experiments in different years, cultivars and environmental conditions. 

Material and methods

Trials were carried out in 3 commercial organic vineyards of Marche Region, Central-Eastern Italy, along 3 seasons (since 2019 to 2021). Treatments were applied on cv Verdicchio of wineries Moncaro (in Castelplanio, AN) and Belisario (in Matelica, MC), and on ‘Montepulciano’ of winery Moncaro, in Angeli di Varano (AN). Chitosan was applied at 0.5% along the season, in the second half of the season following copper, and combined at half rate (0.25%) with half copper rate. Untreated and copper treated plots were used as a control. Downy mildew infections were recorded along the season, and disease incidence, severity and McKinney index were calculated.

Results

The weather conditions were variables among the years and the vineyards. Chitosan treatments reduced McKinney Index of downy mildew, compared to untreated control, on bunches of cv. Verdicchio by 41.7%, 85.5%, and 43.9%, and on cv. Montepulciano by 17.1%, 68.9%, 45.2%, respectively, in 2019, 2020 and 2021. Similar results were obtained on leaves. In 2021, combination of chitosan with reduced copper rates protected leaves and bunches from downy mildew at the same level of copper. This investigation demonstrates that chitosan can be introduced in downy mildew management strategies and can integrate or even replace copper with similar degree of protection and with lower amounts of residues on the grapes.