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 a major source of income for smallholders in the Ecuadorian Andes, where fungal root and foliar diseases reduce yield and commercial quality and are controlled mainly with single-site fungicides. We evaluated the in vitro sensitivity of four fungal isolates (Fusarium sp. SFCBDMR.01, Penicillium sp. SFCBDMR.24, Alternaria sp. SFCBDMF.04, and Ulocladium sp. SFCBDMR.13) obtained from symptomatic white onion in Santa Fe de Galán, Ecuador, and pathogenic on this host (Koch’s postulates), to 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, incorporated into potato dextrose agar at 0–500 µg mL⁻¹. Sensitivity was quantified as the mycelial growth rate and the median effective concentration (EC₅₀), estimated using a three-parameter log-logistic model. Tebuconazole was highly active against Fusarium sp. (EC₅₀ = 0.24 µg mL⁻¹) and Penicillium sp. (EC₅₀ < 0.1 µg mL⁻¹) and moderately active against Alternaria sp. (7.41 µg mL⁻¹); mefentrifluconazole was highly active against Alternaria sp. and Ulocladium sp., and thiabendazole against Ulocladium sp. (EC₅₀ < 0.1 µg mL⁻¹). The QoIs and thiophanate-methyl did not halve the mycelial growth of most isolates (EC₅₀ > 500 µg mL⁻¹), except Alternaria sp. with pyraclostrobin (36.0 µg mL⁻¹) and Penicillium sp. with azoxystrobin, which showed a concentration-independent inhibition plateau of 61–64%. Responses were specific to each isolate–active ingredient combination, even within a FRAC group. Because one isolate per genus was tested and no baseline population was available, these values describe phenotypic sensitivity rather than resistance. They are, to our knowledge, the first fungicide sensitivity reference values for fungi associated with white onion in Ecuador and provide a basis for selecting active ingredients and for future resistance monitoring in this crop.