Baseline Sensitivity Profiling of Alternaria mali Populations to Tebuconazole in Apple (Malus sp.), a Demethylation Inhibitor Fungicide

In Jammu and Kashmir, apple farming has evolved into an industry, contributing around 65% of the country's total production and serving as a cornerstone of the state's economy. However, several diseases caused by fungal pathogens significantly reduce crop yield and quality. Demethylation inhibitor fungicides viz., tebuconazole, difenoconazole, myclobutanil, hexaconazole and flusilazole are a key group of systemic fungicides recommended for use on apples in the valley to manage Alternaria mali by inhibiting the synthesis of ergosterol, a critical component of cell and mitochondrial membranes in the pathogen. However, the possibility of these fungicides to rapidly acquire resistance and the recent finding of decreased Tebuconazole sensitivity in Alternaria mali emphasizes the need for Alternaria mali to be tested against tebuconazole in the the valley. Thirty baseline isolates of Alternaria mali were tested at six different concentrations of tebuconazole (0.0001, 0.001, 0.01, 0.1, 1.0, and 10 µg/ml) using mycelial growth assays. BP isolates exhibited high relative growth at lower fungicide doses (65.9–95.9% at 0.0001 µg ml⁻¹), which declined progressively with increasing concentration, resulting in strong growth suppression at 10 µg ml⁻¹ (1.89–34.9%). The mean EC50 value for Tebuconazole was determined to be 0.15µg/ml, and a dose of 0.20 µg/ml was chosen as the discriminatory dose for assessment of resistance in exposed population.