In Vitro Sensitivity of Four Fungi Isolated from Symptomatic White Onion (Allium fistulosum L.) to DMI, QoI, and MBC Fungicides

White onion (Allium fistulosum L.) is an economically important crop whose productivity and commercial quality can be affected by fungal diseases. This study evaluated the in vitro sensitivity of Fusarium sp., Penicillium sp., Alternaria sp., and Ulocladium sp. to six fungicides with different modes of action: the demethylation inhibitors (DMIs) tebuconazole and mefentrifluconazole, the quinone outside inhibitors (QoIs) pyraclostrobin and azoxystrobin, and the methyl benzimidazole carbamates (MBCs) thiophanate-methyl and thiabendazole. Fungicides were incorporated into potato dextrose agar (PDA) at concentrations of 0, 0.1, 1, 10, 100, and 500 ppm, and sensitivity was assessed based on mycelial growth rate (MGR) and median effective concentration (EC₅₀). Fungicide concentration significantly affected MGR (p < 0.05) in most fungus–fungicide combinations, although the magnitude of the response varied among fungal isolates and active ingredients. Fusarium sp. was highly sensitive to tebuconazole (EC₅₀ = 0.10 µg mL⁻¹), thiophanate-methyl (0.02 µg mL⁻¹), and thiabendazole (0.11 µg mL⁻¹), whereas Alternaria sp. showed high sensitivity to mefentrifluconazole (0.019 µg mL⁻¹), azoxystrobin (0.01 µg mL⁻¹), and pyraclostrobin (0.04 µg mL⁻¹). Penicillium sp. was highly sensitive to tebuconazole (0.07 µg mL⁻¹) and azoxystrobin (0.06 µg mL⁻¹), but showed markedly reduced sensitivity to mefentrifluconazole (749.53 µg mL⁻¹). Ulocladium sp. exhibited variable responses, ranging from high sensitivity to azoxystrobin (0.10 µg mL⁻¹) and thiabendazole (0.32 µg mL⁻¹) to moderate sensitivity to tebuconazole (7.46 µg mL⁻¹), pyraclostrobin (8.81 µg mL⁻¹), and thiophanate-methyl (6.90 µg mL⁻¹). Overall, the results demonstrated substantial fungus–fungicide-specific variation, including differences between active ingredients belonging to the same FRAC group. These findings provide baseline information for fungicide sensitivity monitoring and support the rational rotation of fungicides with different modes of action for sustainable disease management in white onion production.