Innovative Delivery of Cu(II) Ions by a Nanostructured Hydroxyapatite: Potential Application in Planta to Enhance the Sustainable Control of Plasmopara viticola

Downy mildew caused by Plasmopara viticola is probably the most serious disease affecting grapevine (Vitis vinifera), and it is capable of causing consistent yield losses. In organic viticulture, the only acceptable and effective means to control the disease is by applications of copper-based fungicides. However, the use of copper in agriculture is expected to be further restricted by European countries because of its critical ecotoxicological and phytotoxicological profile. Research on ways to reduce the effective amounts of copper by developing innovative formulations as well as optimization of the distribution and persistence of copper-based pesticides for downy mildew control seems to be a promising approach. This research investigated the delivery properties of biomimetic synthetic hydroxyapatite (HA) to enhance the biological activity of Cu(II) ions. To this aim, four Cu(II) compounds were formulated with the innovative HA component and applied in an in vitro antifungal assay against Botrytis cinerea, a common grapevine pathogen suitable for in vitro activity tests, and finally, in in planta efficacy assays against P. viticola under greenhouse conditions. The in vitro results highlighted a different inhibition activity for each Cu(II) compound and indicated a different interaction between the cupric compounds and HA, potentially related to different delivery mechanisms of Cu(II) from HA. Under greenhouse conditions, additional findings on the biological activity of the applied formulations were gained, especially on the efficacy of various concentrations of HA in the formulations, the influence of dose variation of the formulation and the treatment efficiency, and the persistence under rain-washing effect. This study revealed promising findings on the formulation based on the HA particles and the soluble Cu(II) compound, which resulted in reduced disease severity and incidence in all of the experimental conditions, including the lower Cu(II) dosage and the rain-washing effect. This suggests that coformulation of the three insoluble Cu(II) compounds with HA might significantly enhance the adsorption and release of Cu(II) ions by HA particles.