Sensitivity assay of Alternaria alternata tangerine pathotype from China to tebuconazole and difenoconazole using resazurin-based microtiter

Alternaria brown spot (ABS), caused by Alternaria alternata tangerine pathotype, is one of the main diseases of tangerine, the hybrid of tangerine and grapefruit, and the hybrid of tangerine and orange. Currently, chemical control is the most effective measurement for this disease management. In this study, 54 isolates of A.alternata tangerine pathotype collected from China was selected for the evaluation of EC50 values to two triazole fungicides (tebuconazole and difenoconazole) by using a rapid resazurin-based microtiter assay. The best assay system is as follows:complete medium (CM), 40 μmol/L final concentration of resazurin (RZ), and 105 conidia/mL spore concentration. The EC50 value for tebuconazole and difenoconazole to inhibit the growth of A.alternata tangerine pathotype population ranged from 0.304 to 0.885 μg/mL and from 0.367 to 0.873 μg/mL, respectively, and averaged 0.562 μg/mL and 0.609 μg/mL, respectively. The frequency distributions of EC50 values of 54 isolates to tebuconazole and difenoconazole were unimodal. Therefore, the mean EC50 values could be used as the baseline sensitivities of A.alternata tangerine pathotype population from China. The determination of the baseline sensitivities of A.alternata tangerine pathotype population to these two triazole fungicides laid the foundation for the selection of fungicide in practice of ABS control and for the future monitoring of the development of the fungicide-resistant subpopulation.