Upcycling Residues from Salvia rosmarinus Distillation and Agroforestry Processes into a Dual-Function Bioagrochemical with Biostimulant and Antifungal Properties

There is a growing demand for sustainable crop strategies that reduce the use of agrochemicals while improving productivity. This study investigates distillation residues of Salvia rosmarinus to develop a novel biostimulant with enhanced antifungal activity, together with natural additives that ensure homogeneity and water dispersibility. Accordingly, several residual by-products released from agroforestry processes and regarded as “plant protection products” were evaluated in phytotoxicity tests in seeds of Lactuca sativa and Lolium perenne. The tested substances included solvents (ethanol, glycerol, propylene glycol, and DMSO) and adjuvants (soy lecithin, polysorbate-20, acetic acid, and ascorbic acid). Those showing the lowest adverse effects were combined with the extract following a 32 factorial design. Most formulations exhibited good water dispersibility and significantly enhanced the germination index of Lactuca sativa, while simultaneously reducing the growth of Lolium perenne and the fungus Aspergillus flavus, all in a clear dose–response manner, as suggested by the four-parameter log-logistic (log10(x)) models. These results indicate that both stimulatory and inhibitory effects were strongly influenced by concentrations, highlighting the importance of optimizing application doses. Among the evaluated carriers, the one lacking glycerol and containing high ratios of polysorbate-20 and soy lecithin demonstrated the most balanced overall performance in terms of physical stability, dispersibility, and biological activity, a response that can be attributed to the combined contribution of the extract and the selected carrier components. Overall, this study demonstrates that bioagrochemicals derived from agroforestry by-products can provide dual-function agricultural applications (biostimulant/antifungal or herbicide/antifungal), while supporting the framework of the circular bioeconomy.