Enhancing the Efficacy of Natural Repellents Against Grapevine Pathogens by Tannins-Lignin-Mixed Nanovectors

The severe use of conventional pesticides has led to rethinking agriculture protocols for crop protection. In this context, attention has been given to nanopesticides, i.e., formulations containing nanosized particles to deliver poorly soluble bioactive compounds. The aim of this work was to design and prepare nanoparticles from biopolymers such as lignin and tannins to allow the encapsulation and transport of neem oil and capsaicin against three grapevine phytopathogenic fungi: Verticillium dahliae, Phaeomoniella chlamydospora, Phaeoacremonium minimum. Tannins from grape seeds were chosen as adjuvants for the lignin matrix forming the nanocapsules to improve compatibility between the nanovectors and the target since tannins are intrinsic components found in vine plants. Neem oil was used as dispersant for the non-polar bioactive substance capsaicin against pathogenic fungi and for its own antimicrobial properties. The size and structure of the particles in these new formulations were characterized prior to in vitro tests. Scanning electron microscopy (SEM) showed that submicrom globular structures constituted the most abundant population. From dynamic light scattering (DLS), it was found that the average diameter in solution was in the range 250-300 nm for loaded vectors and zeta potential (ZP) showed that all the scattering objects had a negative surface charge (in the range from - 52 to - 37 mV). Small angle X-ray scattering (SAXS) was used to get finer insight into the structural properties of plain and loaded aggregates by fitting the intensity diagrams with a superposition of different contributions, which depended on the specific formulation, in agreement with the SEM pictures taken on the solid obtained from solvent evaporated samples. Regarding the antifungal activity, the most promising results were obtained against the fungi P. minimum. In this case, the advantage obtained by administration through nanocapsules was a dramatic reduction in the amount of both neem oil and capsaicin needed for the treatment. The antifungal effect was suggested to stem out from the synergistic activity of the two bioactive compounds.