On the Applicability of Chitosan Oligomers-Amino Acid Conjugate Complexes as Eco-Friendly Fungicides against Grapevine Trunk Pathogens

In a context in which the incidence and severity of grapevine fungal diseases is increasing as a result of both climate change and modern management culture practices, reducing the excessive use of phytosanitary products in viticulture represents a major challenge. Specifically, grapevine trunk diseases (GTDs), caused by several complexes of wood decay or xylem-inhabiting fungi, pose a major challenge to vineyard sustainability. In this study, the efficacy of chitosan oligomers (COS)–amino acid conjugate complexes against three fungal species belonging to the <i>Botryosphaeriaceae</i> family (<i>Neofusicoccum parvum</i>, <i>Diplodia seriata</i>, and <i>Botryosphaeria dothidea</i>) was investigated both in vitro and in planta. In vitro tests led to EC₅₀ and EC₉₀ effective concentrations in the 254.6−448.5 and 672.1−1498.5 µg·mL⁻¹ range, respectively, depending on the amino acid involved in the conjugate complex (viz. cysteine, glycine, proline or tyrosine) and on the pathogen assayed. A synergistic effect between COS and the amino acids was observed against <i>D. seriata</i> and <i>B. dothidea</i> (synergy factors of up to 2.5 and 2.8, respectively, according to Wadley’s method). The formulations based on COS and on the conjugate complex that showed the best inhibition rates, COS−tyrosine, were further investigated in a greenhouse trial on grafted vines of two varieties (”Tempranillo” on 775P and “Garnacha” on 110R rootstock), artificially inoculated with the mentioned three <i>Botryosphaeriaceae</i> species. The in planta bioassay revealed that the chosen formulations induced a significant decrease in disease severity against <i>N. parvum</i> and <i>B. dothidea</i>. In summary, the reported conjugate complexes may be promising enough to be worthy of additional examination in larger field trials.