Evaluation of the Antifungal Potential of Different Photorhabdus Species Against Monilinia laxa and Colletotrichum fioriniae

<i>Monilinia laxa</i> and <i>Colletotrichum fioriniae</i> are major fungal pathogens causing brown rot and anthracnose in stone fruits and shell fruits, leading to significant economic losses. Chemical fungicides are widely applied but can result in resistance development, environmental contamination, and food safety concerns. Biological control using entomopathogenic bacteria (EPB) of the genus <i>Photorhabdus</i> has emerged as an eco-friendly alternative. This study evaluated the in vitro antifungal activity of selected <i>Photorhabdus</i> species (<i>P. kayaii</i> 1723B, <i>P. temperata</i> 3017, <i>P. cinerea</i> 3086, <i>P. laumondii</i> 3196, and <i>P. thracensis</i> 3210) against <i>M. laxa</i> (M3) and <i>C. fioriniae</i> (VV081) using drop-to-drop confrontation and poisoned agar assays. Effects of fermentation time, preparation mode (original vs. centrifuged and filtered), and concentration (5, 10, 20%) were examined. Species-specific inhibition was observed, with Median Inhibition Index values indicated relatively higher antifungal activity for <i>P. thracensis</i> 3210 against <i>M. laxa</i> (0.718) and <i>C. fioriniae</i> (0.552), followed by <i>P. cinerea</i> 3086 (0.643 and 0.552) and <i>P. kayaii</i> 1723B (0.629 and 0.541). Fermentation time and preparation mode influenced antifungal activity in a strain-dependent manner, with longer fermentation periods and original culture preparations generally showing stronger inhibitory trends. Higher concentrations, especially 20%, were often associated with increased inhibition, although the magnitude of these effects varied among strain-pathogen combinations. Overall, these findings demonstrate that the strain- and pathogen-specific nature of antifungal responses in <i>Photorhabdus</i>, supporting their potential as components of targeted biological control strategies rather than uniform broad-spectrum agents.