Invasive Goldenrod (Solidago gigantea Aiton) as a Source of Natural Bioactive Antimicrobial, Insecticidal, and Allelopathic Compounds

Goldenrod (<i>Solidago gigantea</i> Aiton) is a highly invasive species in Europe (e.g., Poland, Germany, and the Czech Republic) whose secondary metabolites can serve as potential sources of bioactive compounds. This study evaluated the phytochemical profile of <i>S. gigantea</i> extracts and evaluated their antibacterial, insecticidal, and phytotoxic activities. The extracts were found to be rich in flavonoids (TFC = 101 mg QE/g) and phenolics (TPC = 175 mg GAE/g), with chlorogenic acid and rutin as dominant constituents. Strong antibacterial activity was observed against Gram-positive bacteria, particularly <i>Staphylococcus</i> spp. (MIC₉₀ = 2.3 mg/mL; MBC = 5 mg/mL), while Gram-negative bacteria were less sensitive, with moderate susceptibility in <i>Rhizobium radiobacter</i> and <i>Pseudomonas syringae</i>. The extract exhibited fungistatic activity against all tested filamentous fungi, with <i>Fusarium</i> species being the most sensitive (49-56% growth inhibition at 10 mg/mL). Insecticidal assays demonstrated significant mortality of <i>Tribolium confusum</i> adults at 2.5-7.0 mg/mL and feeding inhibition at concentrations as low as 0.5 mg/mL. Seedling growth tests showed dose-dependent effects-from mild suppression to moderate stimulation, varying by plant species. Foliar application revealed both stimulatory and inhibitory effects, with the strongest biomass reduction in cress at 10 mg/mL (-45%). These findings indicate that <i>S. gigantea</i> extracts possess potent antibacterial, antifungal, insecticidal, and allelopathic activities. Their concentration-dependent effects on pathogens and plants highlight potential applications in sustainable agriculture, including natural crop protection and integrated pest management.