A polymer-based gel fumigator, having a combination of Monarda citriodora essential oil (MEO) and hexanal: A solution for the postharvest rot and phytotoxicity of apple during storage

Apples are susceptible to various fungal infections following harvest, resulting in significant losses. This study aimed to develop an essential oil based gel formulation to control the postharvest rot of apples caused by Aspergillus foetidus CBS121.28 and increase their shelf life during storage. In vitro, minimal fungistatic and fungicidal doses of MEO and constituents acting in the vapour-phase were established. MFC (Minimum Fungicidal Concentration) for MEO was 1153.85 µL L−1 air, hexanal 384.62 µL L−1 air, and carvacrol 96.15 µL L−1 air. The combinatorial treatment of MEO and constituents indicated synergy (FICI 0.375) between MEO (144.23 µL L−1 air) and hexanal (96.15 µL L−1 air). In contrast, the combination of MEO and carvacrol was antagonistic. In vivo treatment of MEO and hexanal combination on apple fruit by direct fumigation resulted in phytotoxicity. However, the phytotoxicity was remediated using the gel formulations incorporating various concentrations of MEO and hexanal combinations (Carrageenan guar gum gel and soft gel), which showed a synergistic effect (FICI 0.25 and 0.375, respectively). The gel formulations reduced the weight loss, suppressed the degradation of ascorbic acid, and prevented the change of pH and total soluble solids of the stored apples. The antioxidant activity and total phenolic content also increased in the stored apples. These findings suggest that using MEO and hexanal in a gel formulation is a more effective and organic way to protect apples from Aspergillus rot for an extended storage period.