SUMMARY On-farm surveys were conducted in the Uttar Pradesh (India) during the two rainy seasons 2007 and 2008 to monitor pearl millet (Pennisetum glaucum) downy mildew incidence. Twenty-one isolates of Sclerospora graminicola, the pearl millet downy mildew pathogen, were collected from different hybrid cultivars. These isolates were estab- lished on seedlings of the highly susceptible line 7042S grown in the greenhouse and were characterized for their virulence diversity using a set of seven host differential lines. Quantitative differences in virulence among pathogen isolates were determined by calculating viru- lence index (percent disease incidence × latent period -1 ). Results were submitted to cluster analysis using the Aver- age Linkage method to determine similarity among pathogen isolates. The two highly virulent isolates, Sg 492 from Aligarh and Sg 510 from Badaun, representing geo- graphically diverse locations were selected for use in greenhouse screening of pearl millet breeding lines. Key words: Sclerospora graminicola, downy mildew, pearl millet, virulence, pathotype, breeding. INTRODUCTION Downy mildew (DM) caused by Sclerospora gramini- cola is an important disease of pearl millet [Pennisetum glaucum (L.) R. Br.]. The disease is highly destructive and widespread in the major pearl millet-growing areas of the world (Williams, 1984; Jeger et al., 1998). In In- dia, the disease is quite severe on single-cross F 1 hybrids and causes substantial yield losses. S. graminicola is largely heterothallic and has a rapid asexual generation cycle, thus it can produce millions of spores in a short time (Idris and Ball, 1984; Michelmore et al., 1982). Ex- istence of mating types and their frequency greatly con- tributes towards the development of new recombinants in the pathogen populations (Pushpavathi et al., 2006a). Corresponding author: R. Sharma Fax: +91.30713074 E-mail: r.sharma@cgiar.org These characteristics help the pathogen to produce new variants with changed virulence that can be selected on resistant cultivars. Pathogen populations with new viru- lence genes could arise either because of sexual recom- bination/mutation or because of gene flow. Evolution of host-specific virulences in pearl millet downy mildew is well documented (Thakur et al., 1992; Sastry et al., 2001; Pushpavathi et al., 2006b), as a result of which re- sistant genotypes lose their effective resistance within a short period, leading to the development of new patho- types/races in the pathogen populations (Kolmer et al., 2006). Pathogenic variability in S. graminicola studied through a collaborative International Pearl Millet Downy Mildew Virulence Nursery has revealed differences in S. graminicola populations at different locations, including those within India (Thakur et al., 2004, 2006; Rao et al., 2005). The on-farm DM surveys during 1994-2004 in the hybrid-intensive states of Maharashtra, Rajasthan, Gu- jarat, and Haryana revealed increased DM susceptibility of a hybrid when grown in the same field for more than three consecutive crop seasons, indicating the emergence of a pathotype with new/changed virulence (Thakur et al., 2003; Rao et al., 2005). Therefore, pathogen popula- tions in the major crop-growing areas need to be periodi- cally monitored and characterized to identify new patho- types in the target area. Virulence change in S. graminico- la populations is monitored through on-farm surveys for downy mildew incidence and by characterizing pathogen isolates for virulence diversity. Studies done at ICRISAT and elsewhere have shown large pathogenic variability of S. graminicola populations from India and other coun- tries (Shivaramakrishna et al., 2003; Thakur et al., 2004; Sharma et al., 2010). S. graminicola isolates from the major pearl millet- growing areas of India have been characterized and the most virulent pathotypes from different regions are be- ing maintained at ICRISAT for greenhouse screening of breeding material. However, pathogen populations from Uttar Pradesh, the fourth largest pearl millet pro- ducing state (about 0.9 million ha annually) in India, have not yet been characterized. This study aimed at determining the variability in pathogenicity (ability of a pathogen isolate to cause Journal of Plant Pathology (2011), 93 (1), 71-78 Edizioni ETS Pisa, 2011 71 VIRULENCE DIVERSITY IN NORTH INDIAN ISOLATES OF SCLEROSPORA GRAMINICOLA, THE PEARL MILLET DOWNY MILDEW PATHOGEN R. Sharma 1 , V.P. Rao 1 , S. Senthilvel 1 , S.C. Rajput 2 and R.P. Thakur 1 1 International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, 502324 Hyderabad, Andhra Pradesh, India 2 Regional Research Station, Chandra Shekhar Azad University of Agriculture and Technology, 202125 Kalai, Aligarh, Uttar Pradesh, India