J Mycol Pl Pathol, Vol. 40, No.1, 2010 14 PI Award I 2009 Profiling of Phytophthora infestans Populations for Metalaxyl Resistance and Its Management With Novel Action Fungicides Ratinderbir Kaur, T. S. Thind and Sanjay Goswami Department of Plant Pathology, Punjab Agricultural University, Ludhiana 141004. Email: thind_ts@yahoo.co.in Abstract Late blight, a devastating disease of potato, is mainly managed by the use of fungicides as host resistance to Phytophthora infestans is highly unstable. Metalaxyl-based formulations are commonly used by the farmers to get effective control of the disease under congenial weather conditions. Monitoring of sensitivity levels of P. infestans populations from various locations in Punjab state has revealed the prevalence of metalaxyl resistance to varying levels. Characterization of resistant strains based on pathological and molecular parameters was carried out. Among 48 sporangial populations of P. infestans studied, eight populations showed a resistant response at 100 μg/ml of metalaxyl, while two other populations showed a highly resistant response and could infect leaves treated with 200 μg/ml of metalaxyl with a resistance factor of 60 that was calculated as a ratio of ED 90 values of resistant to susceptible population of P. infestans. Resistant strains were found to be highly pathogenic in nature and exhibited competitive fitness in a mixture with sensitive populations. There was no cross resistance to novel action fungicides such as mandipropamid, cymoxanil, benalaxyl, previcur, fluopicolide, azoxystrobin and multisite contact fungicides. Molecular markers were identified for resistant strains using RAPD primers and cluster analysis. Metalaxyl resistance was effectively managed under field conditions through application of novel action fungicides such as Infinito 68.75 SC (fluopicolide+ propamocarrb chloride), Amistar 25 SC (azoxystrobin), Acrobat 50 WP (dimethomorph), Mandipropamid 250 SC and Curzate M-8 72 WP (cymoxanil + mancozeb). Combination of fungicides with different modes of action retards development of resistance and ensure sustainable management of late blight. Key words: Late blight, Phytophthora infestans, metalaxyl resistance, cross resistance, novel action fungicides, resistance management Citation: Kaur R, Thind TS and Goswami S. 2010. Profiling of Phytophthora infestans populations for metalaxyl resistance and its management with novel action fungicides. J Mycol Pl Pathol 40(1):14-21. Late blight of potato (Phytophthora infestans (Mont) de Bary) causes heavy yield losses under favourable weather conditions. Since host resistance to P. infestans is not easily available or is unstable, application of fungicides is an important method to manage late blight of potato. In the event of high disease risk due to congenial weather conditions, phenylamide fungicides, notably metalaxyl-based formulations are frequently used by the farmers in India to get effective control of the disease (Thind et al 2001). Continuous and irrational use of site- specific fungicides is known to be risky and may lead to development of resistant strains in the pathogen populations (Dekker and Georgopoulos 1982). The emergence of resistant strains is quite rapid with metalaxyl and has caused disease control failure in several countries (Davidse et al 1981, Schwinn and Margot 1991, Dowley and O’ Sullivan 1985). Resistance to metalaxyl in P.infestans was first reported in India in Nilgiri hills by Arora et al (1992) and subsequently in Punjab State by Thind et al (2001). However, detailed studies with larger pathogen populations are lacking and no systematic work has been done to characterize resistant strains. In view of the large scale use of metalaxyl-based formulations against late blight of potato in North India, it was considered imperative to monitor sensitivity levels of P. infestans populations from various potato growing locations. Apart from defining the sensitivity levels of P. infestans populations, studies were made to characterize resistant strains based on pathological and molecular parameters and identify ways to manage resistance using novel action fungicides. Materials and Methods Collection of P. infestans populations. P. infestans sporangial populations on infected potato leaves were collected from different potato growing areas in Jalandhar, Hoshiarpur, Patiala, Amritsar, Ludhiana and Nawan Shahar districts in the Punjab State during 2005- 06, 2006-07 and 2007-08 crop seasons. The information