Aerial Spray Application of Plant Protection Products for Grapevine Downy Mildew Control: Efficacy and Canopy Deposit Evaluation in Semi-Field Trials

A growing interest in aerial drone applications has led to the European regulatory proposal 2022/0196/COD, which considers aerial spraying in steep-slope vineyards safer for human health and the environment. Nevertheless, disease control in perennial crops by aerial applications remains under-investigated. This study aims to identify suitable Plant Protection Products (PPPs) for aerial application in vineyards and analytical methods to quantify PPP deposits. A standardized protocol for controlling grapevine downy mildew was developed, testing Metalaxyl-M and copper-based fungicides’ efficacy and foliar depositions. As Italian law prohibits aerial application, an Unmanned Aerial Spray System (UASS) constrained to the ground simulated aerial spray. Leaves were sampled on predetermined days after treatment application for both fungicides’ efficacy evaluation and deposit quantification. Metalaxyl-M applied from UASS showed an efficacy comparable to ground sprays at pre- and post-flowering (≈70%), while copper efficacy from UASS was lower (≈47–63%) at each stage. Aerial sprayings resulted in higher deposits in the upper canopy, potentially explaining the lower efficacy of copper fungicides, while Metalaxyl-M’s systemicity partially compensated for the uneven vertical distribution, improving disease control. This study established a methodology for aerial PPP testing in vineyards, further studies are needed to confirm these findings across different years and locations.