Apple scab caused by Venturia inaequalis is one of the most economically important diseases of apple worldwide. Disease management is largely based on the frequent applications of fungicides, but fungicide resistance is a common problem. Demethylation inhibitor (DMI), methyl benzimidazole carbamate (MBC) and quinone outside inhibitors (QoI) fungicides have been commonly used for controlling apple scab for many decades worldwide. Scientists at the Faculty of Agriculture, Plant Protection Department of Ankara University have monitored the development of resistance to four MBC, DMI and QoI group fungicides with different modes of action in V. inaequalis from apple orchards in Turkey. The sensitivity of V. inaequalis from 27 orchards in six provinces was evaluated to thiophanate‐methyl, myclobutanil, difenoconazole and kresoxim‐methyl by mycelium growth assay and molecular tools. Baseline sensitivity was determined in wild isolates of V. inaequalisfrom apple trees that had never been treated with any fungicides. EC50 value for thiophanate‐methyl, myclobutanil, difenoconazole and kresoxim‐methyl was 0.0992, 0.0315, 0.0048 and 0.0192 μg/ml, respectively. Of the 470 single spore isolates evaluated, 93.4% was very highly resistant to thiophanate‐methyl, while 27.5% and 50.2% were resistant and moderately resistant to myclobutanil, respectively. Difenoconazole and kresoxim‐methyl resistance were confirmed in 7.45 and 97% of the isolates tested, respectively. Molecular studies revealed that resistance to MBC fungicide, thiophanate‐methyl resulted from a mutation at codon 198 in β‐tubulin gene, while the G143A mutation of cytochrome b caused resistant to the QoI, kresoxim‐methyl. "The resistance risk to these fungicides should be taken into consideration in the management of apple scab in Turkey. Our results indicated that V. inaequalis population in Turkey possessed high incidence of resistance to myclobutanil, thiophanate-methyl and kresoxim-methyl. Molecular analysis confirmed high incidence of mutation in target genes associated with resistance of pathogen to MBC and QoI fungicides. Thus, apple growers in the provinces developed fungicide resistance should abandon the use of these fungicides. Difenoconazole is still used for scab control, but be careful to delay reduced sensitivity of V. inaequalis. "More complex strategies including the use of fungicides mixture with different modes of action and multi-site fungicides, the limiting of fungicide application numbers in a season and alternative control managements as well as cultural practices must be developed for the management of apple scab. Also, future studies should be performed on sensitivity of V. inaequalis to other fungicides used for control of apple scab," the scientists explained. Source - https://www.freshplaza.com