Profiling difenoconazole and flusilazole resistance, fitness penalty and phenotypic stability in Venturia inaequalis.

Abstract

Apple scab disease causes significant losses in apple crop production. In the north western Himalayas of India, the indiscriminate use of triazole fungicides to manage apple scab has led to the emergence of triazole-resistant strains of V. inaequalis. To investigate the resistance profile in three Venturia inaequalis populations collected from North, South and Central Kashmir, baseline sensitivity assays were conducted on 30 V. inaequalis isolates unexposed to any fungicides. The mean ED₅₀ value and discriminatory dose of difenoconazole and flusilazole were determined to be 0.584, 0.15 µg ml^-1 and 0.018, 0.02 µg ml^-1 respectively. The assessment at these discriminatory doses revealed a major shift in sensitivity against both fungicides. The sequencing of conserved region-2 of CYP51A1 revealed that the resistant isolates have TTT (Phenylalanine) instead of TAT (Tyrosine) codon at position 133. Moreover, the same mutation was observed in some shifted isolates which confirmed that this mutation is not solely responsible for the development of resistance. From linear mixed-model regression analyses, the fitness parameters of resistant isolates were assessed which revealed that except for oxidative stress at 1 mm H₂O₂ (wherein a decreased micro colony growth linearly increases with resistance), there is no fitness cost associated with the development of resistance against difenoconazole and flusilazole. Meanwhile, the resistance against both fungicides is phenotypically stable. Consequently, it is speculated that these populations are unlikely to regain their sensitivity even in the absence of these frequently used fungicides.