Indian Phytopath.fedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 54 (I) : 91-97 (2001) Monitoring for metalaxyl resistance in populations of Phytophthora infestans and their characteristics in Punjab T.S. THIND, LAKHVIR SINGH, C. MOHAN and JOGINDER PAUL Department of Plant Pathology, Punjab Agricultural University, Ludhiana 141 004 ABSTRACT:Base-linestudy ofthree sensitivepopulations of Phytophthora infestans showed mean MIC value as 7.0 /lgl ml for metalaxyl.Out of 68 populations of P. infestans, collectedduring 1996-99from different potato fieldsin Punjab and from two potato areas in U.P.treated with fungicidesincluding Ridomil MZ, majority exhibited normal sensitive response to metalaxyl. Thirty one populations, mostlyfrom Hoshiarpur district, showed mild to severe infection at 10 /lglml,while 12populations collectedmostly during 1998-99showed varying levelsofinfection at 50 /lglml.When tested at higher concentrations, three populations were able to produce symptoms at 100and 200/lglmlthus showingresistant responsetometaiaxyl.The resistance factors ofthe lowtomoderately resistant populations ranged between2.85to28.57. However,most of these populations were wellchecked by Ridomil-MZ at the recommended dose of 0.2 % exceptP-46 which could cause infection at this concentration. Three sensitive populations of P. infestans were able to acquire resistance to 100,200 and 500 /lgl ml ofmetalaxyl, respectivelyunder laboratory conditions. The resistant populations were highly pathogenic and no cross resistance was observed to any of the fungicides tested except ofurace. Key words: Phytophthora infestans, resistance,metaIaxyl,lateblight,potato Late blight caused by Phytophthora infestans is a highly destructive disease on potato and causes heavy yield losses under favourable weather conditions. Phenylamide fungicides (metalaxyl, ofurace, oxadixyl etc.) provide highly effective control of this disease and are generally used in combinations with multisite contact fungicides (dithiocarbamates, copper- based) for avoiding build up of resistant strains of P. infestans. For the past 10 years, Ridomil-MZ (8% metalaxyl + 64% mancozeb) is being widely used in North India, apart from other fungicides, for controlling late blight of potato (Anon., 1999). Continuous and irrational use of these fungicides is known to be risky and may lead to development of resistant strains in pathogen popula- tions (Dekker and Georgopoulos, 1982). The emergence of resistant strains is quite rapid with metalaxyl and has caused disease control failure in some countries (Schwinn and Margot, 1991). Although, Ridomil-MZ is a combination product and is expected to delay the onset of resistance build up, the use of fungicide mix- tures does not guarantee prevention of resistance build up as both the partners act independently (Gisi and Staehle-Csech, 1989). Kepping into view the large scale use od Ridomil MZ against late blight of potato, it was considered desirable to monitor sensitivity levels of P. infestans populations collected from various places in Punjab and Uttar Pradesh. MATERIALS AND METHODS Collection of diseased samples Late blight affected leaf samples of potato were collected from different areas of Hoshiarpur, Jalandhar, Ludhiana, Patiala and Moga districts in the Punjab State during November-January in 1996-97, 1997-98 and 1998-99 crop seasons alongwith information on fungi- cides used, varieties and disease situation. Diseased samples were also obtained from areas around Meerut in U.P. during 1998-99 season. Diseased leaf samples were brought to the laboratory in fresh polythene bags for further studies. The samples were designated as pathogen populations and numbered 'ls P-l, P-2, P-3 and so on. In all, 68 populations of P. infestans were collected during three years. Maintenance and multiplication of P. infestans populations The diseased leaf samples collected from different areas were washed with running tap water to remove fungicide deposits, dust particles and old sporangia. These were kept separately in plastic trays lined with wet cotton pads and covered with moist polythene sheet for a night at 18±I°C in order to get fresh sporulation. These populations were further multiplied by detached leaf technique (Thind et al., 1989a) using leaves of cv.