SUMMARY Seventeen representative genotypes of Ascochyta ra- biei in the Beja region were analyzed for virulence varia- tion. Virulence characterization was based on rating As- cochyta blight disease on eight differential cultivars us- ing the Riahi et al. (1990) scale. Cluster analysis of the virulence revealed five highly virulent phenotypes be- cause at least six of the differential lines were severely infected. Virulence cluster analysis was compared to phylogenetic grouping as determined by Morjane et al. (1997) using RFLP assay with microsatellite probes. The correlation coefficient between the virulence and RFLP similarity matrices was low (0.02). This low asso- ciation indicates that DNA polymorphism is indepen- dent of virulence because of virulence instability in Tunisia probably due to the utilization of non resistant cultivars and the occurrence of sexual reproduction be- tween the isolates. Key words: virulence, Ascochyta blight, host parasite interaction. INTRODUCTION Ascochyta blight, caused by Ascochyta rabiei is a devastating disease of chickpea (Cicer arietinum L.) an economically important crop in the dry area of West Asia and North Africa. The disease occurs in most chickpea growing countries and can result in up to 100% loss of the crop when winter sown crops. In the Mediterranean area, losses are reduced by spring sow- ing but this not favours high production as the crop growth is limited by the onset of the hot dry summer season. Considerable efforts have been directed at con- trolling the disease through the development of resis- tant cultivars (Singh, 1997). However, this resistance is broken by the appearance of new pathotypes (Udupa Corresponding author: S. Hamza Fax: +216.1.799.391 E-mail: hamza.sonia@inat.agrinet.tn and Weigand, 1997a). Therefore understanding the pathogen variability of A. rabiei is essential for genetic resistance breeding. Pathogen variability of A. rabiei has been demon- strated by many authors and occurs in several regions as in North Africa, Middle East, India and USA (Kaiser, 1973; Harrabi et al., 1988, Gowen et al., 1989; Mmba- ga, 1997). Despite differences in aggressiveness within the isolate, significant genotype x isolate interaction shows that variability of A. rabiei populations can be characterized as a specialization of isolates to different cultivars of one host. Such isolates could then be con- sidered as having the same virulence phenotype or pathotype. According to the differential set used, Vir and Grewal (1974) found 10 pathotypes among field isolates from India. Six races (pathotypes) were identi- fied among 50 isolates from Syria (Reddy and Kab- babeh, 1985). Recently using a differential set of three cultivars with different levels of resistance, Weigand and Udupa (1997a) identified three distinct pathotypes with different levels of aggressiveness. A. rabiei identification based on pathogenic charac- terization suffers from several problems: (i) there is a variability in the aggressiveness within the same patho- type (ii) the lack of standardization in the assay proce- dure and rating disease scale leads to inflated estimates of pathotypic diversity and variability (iii) the different differential hosts used to characterize A. rabiei popula- tions are unable to detect new virulence. Some new variants are virulent to the whole set which presupposes a rapid evolution of virulence. This combined with the difficulties of exploring germplasm resistance for host differential completion makes the introgression of new resistance more difficult. Molecular techniques such as RFLP and RAPD have been shown to be reliable tools for the characteri- zation A. rabiei populations (Weizing et al., 1991; Mor- jane et al., 1994). Using a microsatellite sequence (GA- TA) 4 as a probe Udupa and Weigand (1997b) were able to diagnose A. rabiei pathotypes found in Syria. This genetic relationship with virulence supports the view that pathotypes are stable and composed of dis- cernable clonal lineage. Using the combination of mi- Journal of Plant Pathology (2000), 82 (1), 23-28 Edizioni ETS Pisa, 2000 23 PATHOTYPE VARIATION OF THE REPRESENTATIVE GENOTYPES OF ASCOCHYTA RABIEI IN THE BEJA REGION S. Hamza 1 , S. Samir 1 , A. Rebai 1 , R. Salah 1 , G. Kahl 2 and H. Moncef 1 1 Laboratoire de Génétique, Institut National Agronomique de Tunisie, 43 Av. Charles Nicolle, 1082 Tunis, Tunisia 2 Plant Molecular Biology, Biocentre, University of Frankfurt/Main, D-60439 Frankfurt, Germany