Specific adaptation by Mycosphaerella graminicola to a resistant wheat cultivar

Three cultivars of winter bread wheat (Gene, Madsen and Stephens) were each inoculated as seedlings in the greenhouse with seven or eight individual i solates of Mycosphaerella graminicola collected in 1997 from each of the sa me cultivars in the field. Isolates collected from Gene were virulent to al l three cultivars, while isolates obtained from Madsen and Stephens were vi rulent to those two cultivars and, in all but one case, avirulent to Gene. At its release in 1992, Gene was resistant to M. graminicola, as indicated by both field observations and greenhouse tests, but by 1995 its resistance had substantially deteriorated. This indicated that its resistance was ver tical (sensu Vanderplank) or race-specific, and that commercial cultivation of Gene rapidly selected for strains in the local M. graminicola populatio n that were specifically adapted to overcome its resistance.