Relative importance and pathogenic variability of barley net and spot form of net blotch (Pyrenophora teres) in north west and central highlands of Ethiopia

Barley is one of the important cereal crop grown in the highlands of Ethiopia. The production this crop is threatened by barley disease net blotch caused by the fungus Pyrenophora teres is among the diseases that exists in two forms, P. teres f.sp teres and P. teres f.sp. maculate that produce net and spot form symptoms of the net blotch, respectively. The relative importance of the spot type under Ethiopian condition is not yet well understood because this symptom is indistinguishable from the barley spot blotch disease caused by the fungus Cochliobolus sativus. Due to this studies conducted so far on the net blotch considered only the net type during field assessments of the disease. As a result, either the net blotch problem undermined or the spot blotch was exaggerated as the spot type of net blotch may be considered as spot blotch. To ascertain the relative importance of the two forms, field survey were undertaken in three woredas Anglelana Asagert and Debre Birhan Zuria in North Shewa and Ly Gaynt in Soth Gonder Administrative Zones during the 2001/2002 main crop seasons. In each woreda three-peasant association (PAs) and in each PA, five barley fields were selected and assessed for disease incidence and severity. Hundred leaf samples with spot blotch and net type symptoms were collected from each field. The presence and distribution of the spot type net blotch was further assessed in the laboratory. Result indicated that the two forms of P. teres were present in all woredas. The net form of net blotch was highly distributed throughout the surveyed areas with an average incidence and severity of 94.7% and 28.7%, respectively. Analysis of the 2,250 leaf samples with spot blotch symptoms revealed 1,821 (80.9%) of the samples to be P. teres f.sp. maculate, while only 365 (16.2%) C. sativus and 2.9% other Helmintosporium species. There was no significant morphological and cultural variation (p0.05) among the net and spot forms of this pathogen. In the pathogenecity testes, there was highly significant variation (P0.05) in the mean disease ratings of 11 barley cultivars, inoculated with two type isolates of P. teres. Barley genotypes CI4929, CI5401, CI2750, CI 7584, and CI4907-1 were resistant to all 30 isolates of the two forms and CI9820, CI5791 and 9819 were susceptible. The differential reaction with respect to all pathogen isolates tested indicated that isolates were distinct from each other, and were most probably different physiologic races of P. teres. Highly significant variation (P0.05) among isolates of each forms of P. teres in virulence and aggressiveness observed and among fifteen P. teres f. sp teres isolates, four were most virulent and among fifteen P. teres f.sp. Maculate isolates, one was most virulent on four genotypes. The degree of virulence was not location specific. Moreover, the virulence was more frequently observed in P. teres f.sp. were isolates than the P. teres f.sp. maculate. This study documented the wider variability of the P. teres that requires attention in resistance breeding program.