Unlocking the Potential of Bacillus Strains for a Two-Front Attack on Wireworms and Fungal Pathogens in Oat

(1) Background: Oat (<i>Avena sativa</i> L.) is a crop that is widely used in human nutrition, while it also plays an important role in animal husbandry as a high-quality forage crop. However, this crop is particularly susceptible to combined biotic stressors, including insect pests (<i>Agriotes lineatus</i>) and fungal infections (<i>Fusarium</i> spp.). These stresses act synergistically: root damage caused by wireworms increases the plant's susceptibility to fungal infection, while pathogens further limit nutrient uptake and root system development. In recent years, the reduced efficacy of chemical pesticides against both insect pests and fungal pathogens has highlighted the need for alternative strategies in oat protection, leading to an increased focus on developing bacterial bio-inoculants as sustainable and effective biocontrol agents. (2) Methods: This study aimed to identify bacterial strains capable of suppressing wireworms (<i>Agriotes lineatus</i>) and <i>Fusarium</i> spp. in oats, while simultaneously promoting plant growth. Bacterial isolates were screened for key Plant Growth Promoting (PGP) and biocontrol traits, including IAA and siderophore production, phosphate solubilization, and the presence of toxin- and antibiotic-coding genes. (3) Results: The highest insecticidal effect against wireworms was recorded for <i>Bacillus velezensis</i> BHC 3.1 (63.33%), while this isolate also suppressed the growth of <i>F. proliferatum</i> for 59%, <i>F. oxysporum</i> for 65%, <i>F. poae</i> for 71%, and <i>F. graminearum</i> for 15%. The most effective <i>Bacillus</i> strains (with insecticidal and antifungal activity) were identified and tested in two pot experiments, where their ability to enhance plant growth in the presence of insects and fungi was evaluated under semi-controlled conditions. An increase in plant biomass, grain yield, and nitrogen content was observed in oat inoculated with <i>B. velezensis</i> BHC 3.1 and <i>B. thuringiensis</i> BHC 2.4. (4) Conclusions: These results demonstrate the strong potential of both strains as multifunctional bio-inoculants for enhancing oat growth and mitigating the adverse effects of wireworm damage and <i>Fusarium</i> infection.