Monitoring of sensitivity dynamics of Phytophthora infestans to dimethomorph and mandipropamid and screening of the fungicides against potato late blight in fields

In order to clarify the temporal and spatial dynamics in the sensitivity of Phytophthora infestans to dimethomorph, mandipropamid, and control efficacies of the commonly used fungicides against potato late blight in one-crop potato planting regions of Northern China, the sensitivity to dimethomorph and mandipropamid in P. infestans strains collected in Hebei Province, Inner Mongolia Autonomous Region and Jilin Province from 2011 to 2019 was measured with the mycelial growth inhibition test. Also, the control efficacies of seven fungicides including dimethomorph and mandipropamid under the recommended dosages to potato late blight were assayed in the fields by spraying stems and leaves with fungicide suspensions in 2018 and 2019. The results showed the resistance frequency and resistance index of 922 P. infestans strains to dimethomorph over the years were 25.71% to 100% and 0.31 to 0.50, respectively. The resistance frequency of 890 P. infestans strains to mandipropamid over the years was 0 to 1.43% and only one resistant strain was found in 2013, and the resistance index was 0.25 for each year. The resistance frequency and resistance index of P. infestans strains in the three provinces to dimethomorph were 57.19% to 66.56% and 0.39 to 0.42, respectively. The resistance frequency of P. infestans strains in the three provinces to mandipropamid was 0 to 0.36% and the resistance index was all 0.25. The results of field efficacy trials showed the control efficacy of dimethomorph 50% WP 300.0 g a.i./hm² (83.3% to 85.6%) were higher than those of mandipropamid 23.4% SC 140.4 g a.i./hm² (76.9% to 78.6%) and fluopimomide 50% WG 75.0 g a.i./hm² (70.0% to 71.4%). The control efficacies of cymoxanil 20% SC 162.0 g a.i./hm², fluopimomide + dimethomorph 40% SC 240.0 g a.i./hm², cymoxanil + mandipropamid 43% WG 290.3 g a.i./hm², and fluopicolide + propamocarb hydrochloride 687.5 g/L SC 773.4 g a.i./hm² were 79.8% to 86.9%. Thus, dimethomorph singly or in mixtures and mandipropamid in mixtures could be used to control potato late blight in Hebei Province, Inner Mongolia Autonomous Region, and Jilin Province, but it was necessary to monitor the sensitivity dynamics of P. infestans to dimethomorph and mandipropamid, and apply fungicides containing dimethomorph or mandipropamid in mixtures or rotation with the fungicides of different action modes.