Tackling Conifer Needle Cast and Ash Dieback with Host-Derived Microbial Antagonists Exhibiting Plant Growth-Promoting Traits

Needle cast (<i>Lophodermium seditiosum</i> Minter, Staley & Millar) in Scots pine (<i>Pinus sylvestris</i> L.) and European ash (<i>Fraxinus excelsior</i> L.) dieback (<i>Hymenoscyphus fraxineus</i> (T. Kowalski) Baral, Queloz & Hosoya) are among the most destructive forest and tree plantation diseases in Europe, threatening not only targeted plant species but also the whole ecosystem. While considerable research effort has focused on microbial antagonists against ash dieback, comparable investigations into needle cast biocontrol remain virtually absent from the literature. Here, isolated microbial antagonists from European ash and Scots pine were evaluated for their efficacy against respective pathogens. In vitro dual-culture assays revealed bacteria with strong inhibitory effects on pathogen growth, as well as multiple plant growth-promoting traits (PGPTs). It was found that bacteria from the genera of <i>Pantoea</i>, <i>Erwinia</i>, <i>Priestia</i>, and <i>Pseudomonas</i> inhibited the growth of <i>H. fraxineus</i> by ≥70%. Most significantly, our investigation revealed that bacteria isolated from Scots pine, belonging to the genera <i>Pseudomonas</i>, <i>Bacillus</i>, and <i>Priestia</i>, inhibited the growth of <i>L. seditiosum</i> by 50% to 80%, representing one of the first reported bacterial antagonisms for this neglected pathogen. All isolates were positive for at least two PGPTs, primarily due to mineralization of organic phosphate and the production of siderophores. The dual functional traits of isolated bacteria highlight their potential application in integrated forest protection strategies, particularly for the previously overlooked <i>L. seditiosum</i> pathosystem.