Innovative delivery of Cu(II) ions by a nano-structured 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 L.) and 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 the European countries because of its critical ecotoxicological and phytotoxicological profile. Research on ways to reduce the effective amounts of copper by development of innovative formulations as well as optimization of the distribution and persistence of copper-based pesticides for downy mildew control appeared to be a promising approach. The present 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 grade of inhibition by each Cu(II) compound according to the applied dosage and also indicated the delivery role potentially played by HA, especially on the insoluble copper salts. Under greenhouse conditions, further findings on the biological activity of the applied formulations were gained, especially on the efficacy of various concentrations of HA in the formulations, on the influence of dose variation of the formulation and the treatment efficiency, and persistence under rain-washing effect. The present 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 the experimental conditions, including the lower Cu(II) dosage and the rain-washing effect. This suggests that a deeper investigation of the co-formulation process for the three insoluble Cu(II) compounds with HA might significantly enhance the adsorption and release of Cu(II) ions by HA particles.