A non-destructive method for quantifying Cu, monitoring its residues, and downy mildew incidence on grapevine leaves after the application of copper-based antifungals

Perennial crops such as grapevine are often sprayed with copper-based antifungals against one of the most important diseases in viticulture, downy mildew (DM), which is caused by the biotrophic oomycete Plasmopara viticola. Although the European Union has limited the use of copper-based fungicides in organic farming since 2002, the likely negative effects on the environment, human health and wine quality, as well as soil contamination from diffuse sources in some agricultural regions, should be taken into account. The aim of this work, funded by the INTAVIEBIO project (PSR - FVG), was 1) to minimise the copper use by studying the variance of Cu coating on vine leaves and 2) to develop a non-destructive and rapid method to detect Cu on treated leaves using a new chemical probe that can detect the presence of Cu by image analysis. This study was carried out on five organically managed vineyards in north-eastern Italy. Copper-based fungicides were applied to the canopy at different times during two growing seasons. The Cu dose applied to the grapevine leaves ranged from 1.00 to 3.6 kg ha-1. Cu concentration was analysed in the laboratory using ICP-AES (Inductively Coupled Plasma - Atomic Emission Spectroscopy) after the leaves were collected and washed with a 1% nitric acid solution (HNO3). For direct Cu analysis of the leaves, we also used Cuprotesmo™ papers, which are particularly sensitive to the presence of copper salts and turn pink in the presence of the chemical (the colour changes when the paper is moistened). Images of the coloured papers were taken with a smartphone camera. The Cu concentration data obtained from a mobile device application developed in the INTAVIEBIO project and that measured with ICP-AES were compared on the same leaves. The data evidences that this approach (once well developed) can be used as a non-destructive, rapid and minimal knowledge required method for Cu measurement by farm managers. The mean values of accumulated Cu on the leaves ranged from 1.67 to 7.91 (μg/cm2), with different Cu concentration on young and old leaves. Consequently, the effectiveness of Cu protection against DM and the correlation between concentration and disease incidence was evaluated.