In the Czech Republic, demethylation inhibitors (DMIs) are used both as fungicides in controlling phoma stem canker and as growth regulators. This heavy use can result in the development of resistant isolates. A total of 45 and 286 Leptosphaeria maculans isolates were tested in vitro, using the mycelial growth and microtiter plate assays, respectively. The objective was to determine the sensitivity of L. maculans isolates collected in the Czech Republic to the fungicides tetraconazole, metconazole, and prochloraz. The mean EC50 values with the mycelial growth plate method were 1.33, 0.78, and 0.40 µg mL−1 for tetraconazole, metconazole, and prochloraz, respectively. The mean EC50 values for the microtiter plate assay were 3.01, 0.44, and 0.19 µg mL−1 for tetraconazole, metconazole, and prochloraz, respectively. All three fungicides also had high variation factors that may be due to inserts in the ERG11 promoter region. In addition, cross sensitivity among the three fungicides was observed. Overall, the high variation factors and the PCR (polymerase chain reaction) results observed in this study could signify the presence of resistant isolates in L. maculans Czech populations, especially in isolates tested for sensitivity to tetraconazole.

Phoma stem canker, caused by Leptosphaeria spp. is generally managed with demethylation inhibitor (DMI) fungicides in the Czech Republic. However, there have been reports of resistance to DMIs in L. maculans populations. Fungicide resistance management recommends the application of fungicides with different modes of action, either as mixtures or in rotation with other fungicide classes. The objective, therefore, was to evaluate the efficacy of boscalid, dimoxystrobin and a mixture of boscalid and dimoxystrobin, belonging to the succinate dehydrogenase inhibitor and quinone outside inhibitor fungicide groups, respectively, on L. maculans isolates. A total of 41 and 285 isolates were tested using the mycelial growth and microtitre plate assays, respectively. The EC₅₀ values for the mycelial growth plate method ranged from 0.026–0.984 µg/mL and 0.097–1.653 µg/mL for boscalid and boscalid + dimoxystrobin, respectively. For dimoxystrobin, the EC₅₀ range was wide, between 0.053–95.59 µg/mL. The EC₅₀ values for the microtitre plate assay ranged from 0.001 496–0.836 3 µg/mL and 7.3 × 10^–4–2.801 µg/mL for boscalid and dimoxystrobin, respectively. The results also showed that the L. maculans conidia were more sensitive to boscalid and dimoxystrobin than mycelia.