Protective and curative efficacy of prothioconazole against isolates of Mycosphaerella graminicola differing in their in vitro sensitivity to DMI fungicides

BACKGROUND: Septoria leaf blotch is the most important disease of wheat in Europe. To control this disease, fungicides of the 14α-demethylase inhibitor group (DMIs) have been widely used for more than 20 years. However, resistance towards DMIs has increased rather quickly in recent years. The objective of this study was to evaluate, on plants and under controlled conditions, the protective and curative efficacy of the DMI fungicide prothioconazole against three current isolates of M. graminicola, chosen to belong to different DMI-resistant phenotypes. Fungicide efficacy was assessed by visual symptoms and by quantitative real-time polymerase chain reaction (PCR). RESULTS: With a protective fungicide application, prothioconazole was always effective against each isolate. This was in accordance with the EC50 results. However, curative efficacy differed between the isolates. It remained at a good level, between 60 and 70% against one isolate, whereas it was strongly affected by late applications from 7 days post-inoculation with the two other isolates. CONCLUSION: A protective application of prothioconazole in wheat crops could be the best strategy to keep a high efficacy against Septoria leaf blotch.