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

BACKGROUND: Fungicide resistance is a significant issue affecting disease control in crops worldwide. Resistance to demethylation inhibitor (DMI, Fungicide Resistance Action Committee Group 3) fungicides has evolved in Leptosphaeria maculans and is widespread at high levels across Australia. Molecular tools have recently been developed for the ERG11 gene to monitor and quantify DMI resistance in L. maculans populations. Here, these tools were used to: (i) determine the frequency of resistance in a population needed for the fungicide to become ineffective in controlling disease, (ii) test current levels of resistance in Australian populations, and (iii) explore potential fitness penalties in strains with resistance. RESULTS: Glasshouse experiments demonstrate that the frequency of fungicide resistance needed to cause a loss of fungicide efficacy is between 1% and 10%. The frequency of resistance can also increase by up to 34.4% following just one application, but in the absence of a fungicide there is limited decrease in the frequency of fungicide resistance, indicating a lack or limited fitness penalty in ERG11 mutants. In the field, the frequency of DMI resistance in L. maculans populations varied from 1.8% to 30.0%, with varying levels of field failure observed in all but the lowest frequency, but there was a poor correlation between DMI resistance frequency and DMI fungicide efficacy. CONCLUSION: 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.