Graduation date: 2007Presentation date: 2006-12-15The specificity of quantitative host resistance to plant disease has long been a controversial issue. We examined interactions between wheat (Triticum aestivum) and Mycosphaerella graminicola, causal agent of Septoria tritici blotch, to determine whether specific interactions occur between host and pathogen genotypes that could be involved in eroding quantitatively expressed resistance. Pathogen isolates were collected from two moderately resistant wheat cultivars, Madsen and Foote, in the field in 2004 and 2005 and tested on each cultivar in a factorial design in growth chamber and greenhouse experiments. The resistance of Madsen has eroded significantly in Willamette Valley, OR wheat fields, and Foote is a replacement cultivar expressing a higher level of resistance. In all of the experiments, there was a significant isolate source by cultivar interaction, with isolates generally causing more disease on their cultivar of origin. The cultivar Madsen reacted to isolates in a manner typical of quantitative interactions, while the cultivar Foote demonstrated qualitative reactions more typical of the breakdown of a major resistance gene. The two cultivars may have had similar levels of quantitatively expressed resistance in the field upon commercial release, but it appears that the pattern of reaction to pathogen isolates is different. Pathogen populations may have the ability to adapt to both types of genetic backgrounds, suggesting that specific interactions in quantitative systems can lead to the erosion of moderate resistance in the field.