Two Philippine Photorhabdus luminescens strains inhibit the in vitro growth of Lasiodiplodia theobromae, Fusarium oxysporum f. sp. lycopersici, and Colletotrichum spp.

Abstract Background Fungal phytopathogens are one of the leading causes of loss in global food production. Chemical fungicides have always been used to control the phytopathogens to mitigate losses. However, it is widely known that this approach is not sustainable. Thus, it is essential to develop alternative control methods, such as the use of biological control agents. Results This study provided a preliminary data on the efficacy of 2 local Photorhabdus strains, associated with Heterorhabditis indica BSDS and H. indica MAP, against selected post-harvest fungal phytopathogens, Fusarium oxysporum f. sp. lycopersici, Lasiodiplodia theobromae, Colletotrichum musae, and another Colletotrichum sp., by measuring their in vitro inhibitory activity. The Photorhabdus strains were isolated from the hemolymph of Ostrinia furnicalis infected with H. indica BSDS and H. indica MAP and grown selectively on NBTA media. Firstly, the bacterial endosymbionts' generic identity was confirmed through colony PCR based on a Photorhabdus Txp40 toxin-specific marker. Species identity was then elucidated through 16s marker-assisted GenBank mining as P. luminescens, sharing 99.51–99.58% similarity with P. luminescens subsp. akhurstii (Accession no. AY278643.1). Anti-fungal activity was observed by the bioassay experiments, using cell-free culture filtrates (CFCs), obtained from P. luminescens tryptic soy broth suspensions (OD600 = 2.0) amended in PDA medium (25%v/v) based on percentage growth inhibition. The CFCs of P. luminescens BSDS showed a significantly higher suppressive activity against Colletotrichum musae, Colletotrichum sp., and Lasiodiplodia theobromae, with 93.18 ± 0.46%, 74.15 ± 0.54%, and 60.51 ± 2.04% growth inhibition, respectively, while the CFC of P. luminescens MAP showed a significantly higher suppressive activity against F. oxysporum f. sp. lycopersici with 21.87 ± 0.71% growth inhibition. Conclusions The results strongly showed that these strains of Photorhabdus can be promising biological control agents against these fungal phytopathogens. Further extensive research is warranted for the development of these promising biofungicides into a practical, economically viable, and environment-friendly control strategy that can be incorporated into any integrated pest management program