Aggressiveness of Puccinia striiformis f. sp. tritici Isolates in the South-Central United States.

Although stripe rust, caused by Puccinia striiformis f. sp. tritici, has been an occasional problem on wheat in the south-central United States from 1941 until 1999, the disease has been consistently severe in the region since 2000. Furthermore, since 2000, the geographic range of stripe rust in the eastern United States has expanded, and the old population of races has been replaced by a new population. The objective of this study was to determine whether new isolates of the pathogen were more aggressive and better adapted to warmer temperatures than old isolates. In all, 6 old isolates (collected before 2000) and 14 new isolates (collected since 2000) were evaluated at 12 and 18°C for latent period on wheat seedlings and urediniospore germination on Noble agar. At 12°C, old and new isolates had similar latent periods and spore germination percentages. However, at 18°C, new isolates averaged 2 days less for latent period and double the spore germination compared with old isolates. Therefore, the new isolates are better adapted and, thus, more aggressive at warmer temperatures than the old isolates. These differences may have contributed to the severity of recent epidemics in the region and to the expanded geographic range for stripe rust.