Multi-resistance to thiophanate-methyl, diethofencarb, and procymidone among Alternaria alternata populations from tobacco plants, and the management of tobacco brown spot with azoxystrobin

In 2014 and 2015, a total of 151 tobacco brown spot (Alternaria alternata) isolates were collected from Guizhou Province in China to evaluate their resistance to the benzimidazole thiophanate-methyl, the carbamate diethofencarb, and the dicarboximide procymidone. Resistance to thiophanate-methyl and diethofencarb was observed in all isolates. Resistance to all the three fungicides, thiophanate-methyl, diethofencarb, and procymidone was detected at a frequency of 6.0%. The F167Y single mutation in the β-tubulin gene was found to be associated with resistance to thiophanate-methyl,but no mutation was found in the coiled-coil region of the histidine kinase-encoding gene OS1, a fungal gene for dicarboximide resistance. Procymidone applied at the rate of 20 mg l⁻¹ inhibited spot lesion formation on tobacco leaves with an efficacy of 51.7% for the low resistance (LR) isolates and 74.2% for the procymidone-sensitive isolates. Thiophanate-methyl applied at 100 mg l⁻¹, however, slightly promoted the expansion of disease lesions with an efficacy of −7.7%. Azoxystrobin applied at 10 and 20 mg l⁻¹ provided efficacies of 91.1 and 100%, respectively, regardless of whether the isolates were thiophanate-methyl resistant or procymidone-LR. Further studies suggested that azoxystrobin exhibited excellent protective activity and good curative activity against A. alternata in plants. The baseline sensitivity to azoxystrobin was then determined. In the presence SHAM, the mean EC₅₀ values for conidial germination inhibition were 0.49 ± 0.22 (Mean ± SD) mg l⁻¹. Interestingly, no resistance was recovered through UV irradiation or Agrobacterium tumefaciens-mediated mutagenesis. This research indicated widespread resistance to thiophanate-methyl and diethofencarb, low frequency of (6.0%) resistance to procymidone in A. alternata populations from tobacco, and suggested that azoxystrobin could potentially constitute a good alternative for the management of tobacco brown spot disease.