Fitness and Resistance Mechanism of <i>Fusarium graminearum</i> from Fields to Demethylation Inhibitor (DMI) Fungicides in China

Tebuconazole and prothioconazole are triazole sterol demethylation inhibitor (DMI) fungicides widely used for controlling <i>Fusarium</i> head blight (FHB) in the world. In 2023, 3714 <i>Fusarium graminearum</i> isolates were collected from fields of 8 provinces in China, among which 116 (3.12%) isolates and 82 (2.21%) isolates showed low resistance to tebuconazole (Teb-R, resistance factor [RF] = 2.65-7.65) and prothioconazole (Pro-R, RF = 3.85-6.04), respectively. Importantly, 11 isolates (0.30%) showed dual resistance (Teb-/Pro-R). Teb-R, Pro-R, and Teb-/Pro-R isolates displayed fitness costs in vegetative growth, asexual and sexual reproduction, or pathogenicity. There was partial cross-resistance among the DMI fungicides. No mutations were found in <i>CYP51s</i> of Teb-R, Pro-R, and Teb-/Pro-R isolates. Under tebuconazole treatment, the expression of <i>CYP51B</i> in Teb-R isolates and <i>CYP51A/B</i> and <i>ABC4</i> in Teb-/Pro-R isolates was significantly higher than in sensitive isolates. Under prothioconazole treatment, the expression of <i>CYP51A</i> in Pro-R isolates and <i>CYP51A/B</i> in Teb-/Pro-R isolates was significantly higher than that in sensitive isolates.