Resistance Assessment of Alternaria alternata Causing Tobacco Brown Spot to Dimethachlon and Difenoconazole

To understand the resistance level of Alternaria alternata to dimethachlon and difenoconazole in Jilin and Heilongjiang Provinces, the sensitivity of 318 isolates of A. alternate to the two fungicides was determined by the method of mycelial growth rate and the sensitivity baseline to dimethachlom and difenoconazole were established. The resistant isolates and frequency were determined using the method of minimum inhibitory concentration and the levels of resistance risk of different locations were also evaluated. The results showed that the distribution of sensitivity frequency of isolates was a continuous single peak curve, which was close to the normal distribution. Therefore, the average value of EC₅₀ with 2.916 μg/mL could be used as the sensitivity baseline of A. alternata to dimethachlon in both provinces and the resistance frequency was 21.38%. The sensitivity baseline of A. alternata to difenoconazole was 1.198 μg/mL and its resistance frequency was 18.24%. There was different resistance for the isolates to dimethachlon and difenoconazole between Jilin Province and Heilongjiang Province, and the resistance level in each regions was different. This study would lay the foundation of the resistance risk assessment and provide evidence to develop a scientific strategy to control tobacco brown spot disease in Jilin and Heilongjiang Provinces.