Strain-Dependent Antifungal Activity of Photorhabdus-Derived Ethyl Acetate Extracts Against Monilinia laxa In Vitro and in an Artificially Inoculated Apricot Fruit Assay

Brown rot caused by Monilinia laxa is a prominent disease in apricots, and the development of fungicide resistance highlights the need for sustainable biological control strategies. Entomopathogenic bacteria of the genus Photorhabdus produce diverse secondary metabolites with antifungal activity; however, their efficacy may vary substantially among bacterial strains and between laboratory and fruit-based assays. This study evaluated the antifungal activity of ethyl acetate (EtOAc) extracts obtained from three entomopathogenic bacteria (EPB), Photorhabdus cinerea 3086, P. thracensis 3210, and P. kayaii 1723B, against M. laxa using an in vitro and an artificially inoculated apricot fruit assay. The extracts showed marked strain-dependent differences in antifungal activity. In vitro, P. cinerea 3086 and P. thracensis 3210 showed the highest inhibition indices, 50.35% and 45.23%, respectively, whereas P. kayaii 1723B showed only 5.58% inhibition. In the apricot fruit assay, both bacterial strain and extract dose significantly affected disease suppression, with a significant strain × dose interaction. The highest inhibition index was observed for P. cinerea 3086 at the highest tested dose, reaching 64.79%. In contrast, the strong in vitro activity of P. thracensis 3210 was not consistently maintained in the fruit assay, where inhibition at the highest dose was only 8.51%. A significant positive correlation was observed between in vitro and fruit-assay inhibition indices (ρ = 0.50, p = 0.0246) although the moderate correlation indicates that in vitro activity alone may not fully predict efficacy on fruit. Overall, the results demonstrate substantial strain- and dose-dependent variation in the antifungal activity of Photorhabdus-derived EtOAc extracts and highlight the importance of evaluating candidate extracts in both laboratory and fruit-based systems. P. cinerea 3086 represents a candidate for further investigation, including chemical characterization of its active constituents and evaluation under postharvest or orchard-relevant conditions.