Fifty monoconidial isolates of the rice blast fungus, Magnaporthe grisea,belonging to six rep-PCR Pot2 lineages collected from 11 rice cultivars in 14 cities in Guilan province were analyzed for virulence during 1997-99. Differential varieties were international standard cultivars set, indica CO39 with five near-isogenic lines (NILs) for resistance genes in the background of CO39 and twelve selected Iranian cultivars. These isolates were classified into six pathogenic races based on their virulence to eight international differential varieties. These international races belonged to three race groups IA, IC, and IF. The races IC-26, IC-29 and IF-1 were newly identified in Iran and the IC-25 with 33 isolates was being the most common race in Guilan. In this study, the differentiating ability of the set of NILs was evaluated by comparison with those international differentials and Iranian cultivars. The set of five NILs was able to differentiate 50 isolates into two pathotypes (races) in which one isolate represented pathotype ? A ? and 49 were grouped into pathotype ? B ?. However, the differentiating ability of NILs was not sufficient as compared with that of international differential set and Iranian cultivars. Reaction patterns of twelve Iranian cultivars to 50 isolates belonging to different international races (based on their virulence to the international differentials) could be classified into seven pathogenicity forms. Based on all 26 cultivars and NILs as a differential set, the fifty isolates differentiated into 13 pathogenicity forms. The results revealed that the international race system did not fully describe the virulence spectrum of the isolates, since several races could be further differentiated into different pathotypes when Iranian cultivars were taken to be used as differentials. Among Iranian cultivars examined, local cultivars Binam, Hasansaraii, Alikazemi and Domsiah were fully compatible, whereas, Sangtaroom (another local cultivar), Khazar and Nemat ( two improved cultivars) were fully incompatible to all isolates tested. Therefore, the frequency of virulent phenotypes on the 12 Iranian cultivars tested ranged from 0.0 to 1.0. Sangtaroom as an unimproved and local cultivar was incompatible to all isolates, so that its virulence phenotype was 0.0. As a result, due to resistance of this cultivar to all identified races in Guilan province, it can be used as a useful parent in breeding programs in future.
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