Influence of the Extraction Conditions on the Antifungal Properties of Walnut Green Husk Isolates

The green husk is one of the major wastes produced during the walnut (Juglans regia L.) cultivation. However, the results presented in this article may contribute to extension of their usefulness as new and safe plant protection products. The results confirmed the activity of the walnut green husk extracts against plant (Alternaria alternata, Rhizoctonia solani, Botrytis cinerea, Fusarium culmorum, Phytophthora cactorum) and bee (Ascosphaera apis) pathogenic fungi, and showed that the extraction conditions differentiate the properties of the obtained isolates. The increase in the isolation temperature leads to a significant reduction in the antifungal activity of the extracts. The change of the extractant type modifies the resistance of individual fungi. The observed variations in the properties are consistent with the change of juglone content in these extracts. However, considering the juglone concentration provided 50% inhibition of the growth of individual mycelia, the observed differentiation of the antifungal properties of extracts should not be associated only with the juglone content. This observation, supported by the results obtained using gas chromatography coupled to mass spectrometry (GC-MS) and liquid chromatography coupled to mass spectrometry (LC-MS), suggests a synergistic effect of the other components on the antifungal activity of walnut green husk extracts.