Antifungal activity of Xenorhabdus and Photorhabdus against aerial and soilborne grapevine pathogens: varying efficiencies and non-target effects

The extensive use of fungicides raises significant environmental and health concerns, including biodiversity loss and risks to agricultural workers, while the emergence of new fungal diseases exacerbates reliance on these chemical agents. Recent research highlights the potential of Xenorhabdus and Photorhabdus, symbiotic bacteria of entomopathogenic nematodes (EPN), as alternative biocontrol agents against soilborne and aerial fungal pathogens. This study aimed to evaluate the efficacy of EPN-symbiotic bacteria and their by-products against selected fungi and their potential non-target effects on biocontrol agents used in vineyards as a model agroecosystem. Specifically, this study investigated (i) the antifungal effect of Xenorhabdus nematophila cultured in three different nutrient media (Tryptone Soya Broth- TSB-, Nutrient Broth –NB, and Luria-Bertani –LB-) and under varying fermentation durations (3 and 10 days) against Botrytis cinerea; (ii) the antifungal activity of X. nematophila and Photorhabdus laumondii subsp. laumondii against the soilborne pathogen Armillaria mellea; and (iii) the antibacterial activity of X. nematophila, X. bovienii, P. laumondii subsp. laumondii and Photorhabdus luminescens subsp. kayaii by-products against Bacillus spp., including strains used as commercial biocontrol products. Our results demonstrated that X. nematophila cultured in TSB and NB produced the highest inhibition of B. cinerea, with inhibitory effects ranging from 63.5 % to 74.3 %, depending significantly on the fermentation duration. For A. mellea, both cell-free supernatants (CFS) and unfiltered ferments (UFs) from X. nematophila and P. laumondii subsp. laumondii significantly reduced colony numbers and colonized areas, with UFs exhibiting superior efficacy. Non-target effect assays revealed selective antibacterial activity, with significant inhibition observed only against Bacillus thuringiensis var. kurstaki ABTS-351, while no effect was detected against B. thuringiensis PB-54 or B. amyloliquefaciens QST-713. These findings highlight the dual potential of Xenorhabdus and Photorhabdus bacteria as biocontrol agents for fungal pathogens and for promoting selective microbial interactions. However, further studies are needed to optimize their efficacy and assess potential non-target effects in integrated pest management in vineyards and other agroecosystems.