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Specificity of Incomplete Resistance to Mycosphaerella graminicola in Wheat

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 resistance. The moderate resistance of the wheat cultivar Madsen has eroded significantly in the Willamette Valley of Oregon since its release in 1990. Foote is a replacement cultivar expressing moderate resistance and was released in 2000. Isolates of M. graminicola were collected from Foote and Madsen in 2004 and 2005 and tested on each cultivar in growth chamber and greenhouse experiments. There was a significant (P ≤ 0.002) cultivar by isolate source interaction in all experiments when the isolates were tested individually and also when they were tested as bulks of isolates collected from the same cultivar. Though the resistance of Foote is still very effective in the field, isolates sampled from that cultivar at the end of the season in both 2004 and 2005 were of high virulence on Foote, while those collected from Madsen usually were of low virulence on Foote. Foote demonstrated qualitative reactions more typical of a major resistance gene that provides incomplete resistance. Madsen showed a more continuous variation in reaction to M. graminicola isolates. The mean of the isolates collected from Madsen caused significantly (P ≤ 0.05) more disease on Madsen than on Foote for the individual isolates collected in 2005, but not for those collected in 2004. Bulk populations collected from Madsen did not cause significantly more disease on Madsen than did isolates collected from Foote in either 2004 or 2005.