Abstract:Sensitivity to fludioxonil of 122 isolates of Alternaria alternata, causal agent of potato early blight, collected from potato plants growing in the major regions of potato production in Hebei Province was detected using mycelial growth rate test. Sensitivity of 15 UV-induced mutants resistant to fludioxonil and their 3 sensitive parental isolates to fludioxonil, iprodione, procymidone, difenoconazole and pyraclostrobin was tested with mycelial growth rate assay. EC50 values of 122 isolates to fludioxonil ranged from 0.0025 μg/mL to 0.0857 μg/mL with a mean of 0.0404±0.0171 μg/mL, EC50 value of the most insensitive isolate was 34.87 times that of the most sensitive one, and the frequency distribution of the 122 isolates conformed to approximate Gaussian distribution, so the average EC50 value was regarded as the baseline sensitivity of A.alternata to fludioxonil. Besides, no cross-resistance correlation in A.alternata exists between fludioxonil and difenoconazole or pyraclostrobin but positively correlated cross-resistance relationship exists between fludioxonil and iprodione or procymidone. In field trial on controlling efficacy, the control effect of fludioconil 50% WP applied at the rate of 105 or 115 g/hm2 against the potato early blight was all above 80% and significantly higher than that of the conventional fungicides mancozeb 80% WP applied at the rate of 1 350 g /hm2 and propiconazole 25% EC applied at the rate of 37.5 g/hm2. So fludioxonil could be used for control of potato early blight as the substitute fungicide for pyraclostrobin and difenoconazole.