Abstract: 【Objective】 The objective of this study is to clarify the dynamics of resistance to phenylamide fungicide metalaxyl of Phytophthora infestans from potato of Hebei Province, Inner Mongolia Autonomous Region and Jilin Province, screen for highly effective fungicides that can replace metalaxyl, assess the control efficacy against potato late blight of the alternate application of these alternative fungicides, and to provide the clues for control of potato late blight in the regions where P. infestans developed severe resistance to metalaxyl. 【Method】 A total of 634 single cystospore strains of P. infestans were isolated from major production areas of potato of Hebei, Inner Mongolia and Jilin. The sensitivity of 634 P. infestans strains to metalaxyl was determined by the mycelial growth inhibition test. The highly effective fungicides that can replace metalaxyl were screened through the control efficacy trials of fungicides against potato late blight in the fields, the control efficacy of alternate application of alternative fungicides against potato late blight in the area where metalaxy-resistant strains prevailed in P. infestans population was evaluated in the field trials.【Result】P. infestans widely developed resistance to metalaxyl in Hebei, Inner Mongolia and Jilin, the frequency of metalaxyl-resistant strains was up to 100% among 634 P. infestans strains collected from 2011-2016. The average resistance factor and resistance index to metalaxyl was 34 934 and 0.97, respectively. Moderately resistant or highly resistant strains predominated in all strains collected from 2011-2012, whereas the highly resistant strains predominated in all strains collected from 2013-2016. The average resistance factor of the strains from Jilin and Inner Mongolia was higher than that of the strains from Hebei. Resistance index of the strains collected from Jilin and Inner Mongolia in 2011-2012 was higher than resistance index of the strains collected from Hebei in 2011-2012. Resistance index of the strains collected from Hebei, Jilin and Inner Mongolia in 2013-2016 reached the maximum 1.00. In the field trials on control efficacy of fungicides applied 4 times at the recommended dosages, 25% metalaxyl SC, 68% mefenoxam·mancozeb WG, and 64% oxadixyl·mancozeb WP provided the efficacies of 37.0%, 67.5% and 50.9-65.8%, respectively, significantly less effective than 80% mancozeb WP (72.8%) in control of potato late blight. Whereas spray of 10% oxathiapiprolin OD, 50% LH-2010A WG, 50% dimethomorph WP, 687.5 g·L^-1 fluopicolide·propamocab hydrochloride SC, 16% oxathiapiprolin·azoxystrobin SC, or 26% oxathiapiprolin·mandipropamid SC gave good control efficacies (83.9%-90.3%) against potato late blight, undertaking of two alternate application programs consisting of 5 or 6 sprays of the highly effective fungicides (such as 10% oxathiapiprolin OD, 50% LH-2010A WG, 50% dimethomorph WP, 687.5 g·L^-1 fluopicolide·propamocab hydrochloride SC) with mancozeb, azoxystrobin, and flumorph provided good control efficacy of 88.3%-90.5%, and the yield in the districts of rotation application of the highly effective fungicides increased by 64.4%-78.3%, compared with the yield in the districts of rotation application of conventional fungicides (such as mancozeb, phenylamide fungicides and cymoxanil·mancozeb). 【Conclusion】 P. infestans widely developed resistance to metalaxyl in Hebei, Inner Mongolia and Jilin, which led to significant decrease of effectiveness of phenylamide fungicides including metalaxyl·mancozeb, mefenoxam·mancozeb, and oxadixyl·mancozeb. Spray use of the mixed fungicides containing metalaxyl, oxadixyl, or menfenoxam should be suspended in the regions where metalaxyl-resistant strains prevailed in P. infestans population. The fungicides including 10% oxathiapiprolin OD, 50% LH-2010A WG, 50% dimethomorph WP, 687.5 g·L^-1 fluopicolide·propamocab hydrochloride SC, 16% oxathiapiprolin·azoxystrobin SC, or 26% oxathiapiprolin·mandipropamid SC could replace metalaxyl. The contact fungicides with broad spectrum including mancozeb could be used in alternation with the effective systemic fungicides including azoxystrobin, dimethomorph, flumorph, oxathiapiprolin, and fluopicolide·propamocab hydrochloride for highly effective control of potato late blight.

Key words: Phytophthora infestans, metalaxyl, resistance dynamics, alternate application of fungicides, control efficacy