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

Abstract

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the geographic range of stripe rust in the eastern United States has expanded, and the old population of races has been replaced by a new population.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.

Seasonal Changes Drive Short-Term Selection for Fitness Traits in the Wheat Pathogen Zymoseptoria tritici

“…In this context, studies of seasonality enable the collection of relevant epidemiological data and the testing of hypotheses of short-term selection, because it is possible to experiment at the actual time on a small scale. Assessing adaptation by conducting cross-infection experiments, in which the mean fitness levels of sympatric and allopatric fungal populations are compared (59,60), would be useful for predicting responses to thermal selection. This is crucial because recent models and experiments have suggested that the impact of temperature fluctuations on fitness may amplify the impact of climate warming on different species (61).…”

Rust-proofing wheat for a changing climate

“…tritici pathotypes have different temperature optima and virulence profiles than pathotypes collected before 2000 (Markell and Milus 2008). Latent period and spore germination studies indicated that the contemporary population was better adapted to high temperature (Milus et al 2006). Both populations had similar latent periods and spore germination rates at 12°C, but at 18°C, isolates of the new population had shorter latent periods and higher germination rates than the old isolates.…”

“…Both populations had similar latent periods and spore germination rates at 12°C, but at 18°C, isolates of the new population had shorter latent periods and higher germination rates than the old isolates. Such adaptation may have been responsible for the more widespread stripe rust epidemics covering at least 20 central and southern states of the USA in 2000 (Milus et al 2006). Further studies showed that recent severe stripe rust epidemics were most likely magnified by increased pathogen aggressiveness at higher temperature.…”

“…For example, rust reaction of wheat varieties with the stem rust resistance gene Sr15 can change from resistant at 15°C to nearly fully-susceptible at 20°C (Roelfs 1988). Anecdotal evidence of a weakening of stripe rust resistance and pathogen adaptation due to temperature increases has come from annual pathotype surveys in the Eastern USA (Markell and Milus 2008), but these findings may also reflect a change in the pathogen population (Milus et al 2006;Milus et al 2009), described in the following section. Conversely some stripe rust resistance genes, such as Yr18, are known to be temperature mediated and become more effective at higher temperatures (Park et al 1992), therefore in varieties with this resistance there may be an enhancement of the effectiveness of resistance in a warming climate.…”

The Rust Fungi

“…Isolates of P. striiformis f. sp. tritici that are adapted to warmer temperatures have been recently described (Milus and Seyran, 2006). The three wheat rusts generally have slightly different optimum temperature and moisture conditions for infection and preference of tissue to colonize, giving each pathogen different niches on the same host.…”