Eucalyptus globulus Bark Extract and Hinokitiol in Hybrid Anti-Sapstain Formulations with Reduced Chlorothalonil, Carbendazim and Copper-8-quinolinolate Loads

The general objective of this study was to evaluate Eucalyptus globulus bark extract as a candidate component of wood preservatives with a reduced load of synthetic biocides. Extraction conditions were screened using a 23 full factorial design with a center point, identifying 75 °C, 60% ethanol and 1 h as the highest-yielding conditions (5.94%) within the tested ranges. In four-week laboratory wood-block assays, the extract alone did not exceed 40% inhibition of wood-staining fungi, whereas hinokitiol reached complete inhibition at 0.4%. Combining the extract (5%) with hinokitiol (0.4%) achieved 95–100% inhibition, similar to the synthetic reference; because hinokitiol alone reached complete inhibition at this concentration, the extract’s contribution could not be isolated, and formulations in which the extract or hinokitiol was used instead of chlorothalonil and carbendazim alongside a copper-8-quinolinolate product retained 97–100% inhibition at half, and 90–93% at a quarter, of the copper-product dose (87% and 93% reductions in synthetic active ingredients), although the copper product alone was also fully effective at 2% and the contribution of the natural components could not be isolated at that dose. On a per-mass basis, the extract (48 h LC50 27,500 ppm) was approximately 710-fold less acutely toxic than the copper-8-quinolinolate product used as synthetic reference (38.71 ppm) in Daphnia magna bioassays. On the basis of its composition and the literature, the extract may contribute multi-target antifungal mechanisms, including metal chelation, enzyme inhibition and membrane disruption, that could complement the more stable action of hinokitiol; these mechanisms were not measured here. This work supports eucalyptus bark extract, a residue that is abundant in Chile and in other eucalypt-pulp-producing countries, as a candidate component of reduced-synthetic (hybrid) wood-protection formulations; the aquatic toxicity of the final formulations was not tested, and leaching, penetration and field-durability testing are required before practical or environmental benefits can be claimed.