Valorization of <i>Quercus suber</i> L. Bark as a Source of Phytochemicals with Antimicrobial Activity against Apple Tree Diseases

Cork, an anatomic adaptation of the bark of <i>Quercus suber</i> L. through its suberization process, finds its main application in the production of bottle stoppers. Its processing results in a large waste stream of cork fragments, granulates, and dust, which may be susceptible to valorization. The work presented here explored the use of its extracts to inhibit the growth of phytopathogenic microorganisms associated with apple tree diseases. The in vitro antimicrobial activity of cork aqueous ammonia extract was assayed against four fungi, viz. <i>Monilinia fructigena</i> and <i>M. laxa</i> (brown rot), <i>Neofussicoccum parvum</i> (dieback), and <i>Phytophthora cactorum</i> (collar and root rot), and two bacteria, viz. <i>Erwinia amylovora</i> and <i>Pseudomonas syringae</i> pv. <i>syringae</i>, either alone or in combination with chitosan oligomers (COS). Effective concentration values of EC₉₀ in the 675–3450 μg·mL⁻¹ range, depending on the fungal pathogen, were obtained in growth inhibition tests, which were substantially improved for the conjugate complexes (340–801 μg·mL⁻¹) as a result of strong synergism with COS. Similar enhanced behavior was also observed in antibacterial activity assays, with MIC values of 375 and 750 μg·mL⁻¹ for the conjugate complexes against <i>P. syringae</i> pv. <i>syringae</i> and <i>E. amylovora</i>, respectively. This in vitro inhibitory activity was substantially higher than those exhibited by azoxystrobin and fosetyl-Al, which were tested for comparison purposes, and stood out among those reported for other natural compounds in the literature. The observed antimicrobial activity may be mainly attributed to the presence of glycerin and vanillic acid, identified by gas chromatography–mass spectroscopy. In the first step towards in-field application, the COS–<i>Q. suber</i> bark extract conjugate complex was further tested ex situ against <i>P. cactorum</i> on artificially inoculated excised stems of the ‘Garnem’ almond rootstock, achieving high protection at a dose of 3750 μg·mL⁻¹. These results suggest that cork industrial leftovers may, thus, be a promising source of bioactive compounds for integrated pest management.