Virulence of Gaeumannomyces graminis var. tritici from fields under short‐term and long‐term wheat cultivation in the Pacific Northwest, U.S.A.

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Abstract
Gaeumannomyces graminis var. tritici was recovered from 63% of 731 winter wheat plants collected randomly from six sites where wheat had been grown in monoculture for the previous 7–22 years. Typical take‐all was not evident at the time the plants were collected. The fungus was isolated by a baiting method without regard to the presence of take‐all on the plants. Isolates from fields under short‐term wheat cultivation (3 years or less after a break crop) were obtained by plating directly from infected roots of plants with typical take‐all. Virulent isolates comprised 89 and 99% of those collected from long‐ and short‐term wheat cultivation respectively. There was also only a slight difference in the proportions of virulent isolates among monoascosporic subcultures from the two groups of isolates. There was thus little evidence that, during prolonged wheat cultivation, declining virulence in the population of G. graminis var. tritici could account for the absence of take‐all.