Determining frequencies of fungicide resistance in <scp> <i>Leptosphaeria maculans</i> </scp> that render demethylation inhibitor fungicides ineffective in the field

A loss of control of blackleg disease with DMI fungicides can occur when 1-10% of the population is DMI fungicide resistant, with all Australian L. maculans field populations tested being within or above this range. Therefore, for many areas in Australia, applications of DMI fungicides may not provide adequate or expected levels of disease control. Fungicide resistance is not the only determinant of fungicide efficacy and other factors such as infection timing, disease pressure and climate may also contribute. To maintain disease control and high yields, farmers in these areas may need to consider using non-chemical control methods or use other fungicide modes of actions. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.