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

Goldenrod (Solidago gigantea 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 S. gigantea 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 Staphylococcus spp. (MIC90 = 2.3 mg/mL; MBC = 5 mg/mL), while Gram-negative bacteria were less sensitive, with moderate susceptibility in Rhizobium radiobacter and Pseudomonas syringae. The extract exhibited fungistatic activity against all tested filamentous fungi, with Fusarium species being the most sensitive (49-56% growth inhibition at 10 mg/mL). Insecticidal assays demonstrated significant mortality of Tribolium confusum 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 S. gigantea 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.