Sensitivity of Alternaria alternata to Four DMI Fungicides and Resistance Risk Assessment to Mefentrifluconazole

Alternaria alternata often causes preharvest potato early blight and postharvest tuber rot, which can result in substantial yield losses. Demethylation inhibitors (DMIs) are widely used to control Alternaria diseases. However, the underlying resistance risk remains poorly understood. In this study, 114 A. alternata isolates from five major potato-producing regions in Shandong, China, were evaluated for sensitivity to four DMIs (mefentrifluconazole, prothioconazole, tebuconazole, and difenoconazole). The mean EC50 values were 0.622, 6.053, 4.763, and 0.651 mg·L−1, respectively. The Shapiro–Wilk test confirmed unimodal sensitivity distributions, allowing these mean EC50 values to serve as baseline sensitivities. Two-year, two-location field trials demonstrated good control efficacy for all fungicides, with mefentrifluconazole exhibiting the highest efficacy. Three resistant mutants produced by artificial selection in the laboratory exhibited 3.78-, 28.06-, and 15.73-fold higher EC50 values and showed fitness costs, including reduced or comparable mycelial growth, sporulation, and spore germination, pathogenicity similar to that of the parental strain, and lower temperature adaptability. By establishing the first baseline sensitivity of A. alternata from potato to these four DMI fungicides and assessing the resistance risk of mefentrifluconazole, this study provides a basis for rational fungicide application, resistance management, and future field resistance monitoring.