lResearch Institute of Plant Production, Bratislavskd cesta 122, 921 68 Piegfany, Slovakia

Background and objectives
Net blotch of barley caused by the fungus Pyrenophora teres (Died.) Drechs. is a destructive foliar disease world-wide. It may reduce barley production and economic value of the crop. The relative importance of net blotch has increased during the past decade in Slovakia and elsewhere This has happened because predominant barley cultivars currently grown lack satisfactory resistance to net blotch [1]. The pathogen as a facultative saprophyte is characterized by a large genetic variability leading to the formation of numerous pathotypes. Therefore future successful breeding for resistance and making resistant cultivars desirable for effective management requires detailed knowledge of the P. teres population [2]. From this viewpoint the Slovak P. teres population has been analysed.

Materials and methods
The diagnosis of P. teres infection on infected leaves was based on visual assessment and microscopic technique. A method of cultivation on suitable nutrient medium was used to estimate growth characteristics and in vitro sporulation of a large number of monoconidial isolates. Single-spore isolates, set of differentials and laboratory method of testing on detached leaves in benzimidazole solution was used to assess diversity of virulence among Slovak P. teres isolates. Genetic diversity of isolates based on molecular method has been also caried out by RAPD and SSR analysis.

Results and conclusions
Analysis of Slovak P. teres population from a range of sites aimed at symptom expression, culture morphology, virulence and molecular dilterentiation has been carried out. All these parameters studied showed high level of inter and intrapopulation variability. Within the Slovak P. teres population three kinds of symptoms were observed: net type lesion characterized by dark brown blotches crisscrossed with a net-like venation and accompanied by chlorosis; spots of various shapes and sizes encircled by varying width of chlorosis and finally stripe like pattern. When studying characteristics of monoconidial isolates, the considerable variability of morphological expression was recorded and between the dark- and light-coloured isolates highly significant difference in reproduction ability in vitro were also found. P. teres populations from diverse geographical regions of Slovakia underwent virulence analysis. 249 single-spore isolates were obtained and analysed during last three years. Based on responses of the set of differentials (12 barley genotypes) to them virulence diversity was found out and 90 pathotypes were identified. Genotypes of Ethiopian origin CI. 5791, CI. 9819, CI. 9820 and CI. 9825 have been attacked by the lowest percentage of virulent clones and confirmed their stable high resistance not only to the local populations but to the Slovak P. teres population as a whole. On the other hand, Manchurian genotype CI. 739 which exhibited the low frequency of virulent clones to the population collected in 1995 was highly defeated by the populations from 1996 and 1997. Conversely, another Manchurian genotype Tifang (CI. 4407) with moderate resistance to all isolates from 1995 appeared to be high resistant to the populations from 1996 and 1997. The significant differences of aggressiveness among the local populations have been revealed. The most aggressive population manifested a mean of 51.6 % virulent clones. A high level of inter- and intra-population variability of populations has been revealed by amplification of simple sequence repeats (SSR). The (GATA)4 and (GACAGATA)2 sequences used as primers generated multiband polymorphic patterns. Populations isolated from six localities of Slovakia were distinguished each of others. As a result of the analysis of P. teres, populations from Slovakia the genotypes with the stable resistance and wide effectiveness against populations have been found out. For this reason they may have been used for barley resistance breeding to net blotch.

1. Wilcoxson RD, Rasmusson DC, Treefu LM, Suganda T, 1992. Plant Dis., 76, 367-369.
2. Suganda T, Wilcoxson RD, 1993. International Journal of Tropical Plant Disease 11, 147-160.