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Despite the global agro-economic importance of the fungus <i>Rhynchosporium commune</i>, little is known about its evolutionary ecology, including adaptation to different thermal regimes across populations from climatically diverse locations. We conducted common garden experiments with 126 genetically distinct isolates from 9 field populations to measure variation in growth rates at 12°C, 18°C and 22°C. Populations from colder climates with higher temperature variation had higher growth rates at all three temperatures compared to populations from warmer more constant climates indicated by a positive correlation between variance in mean annual temperature and mean population growth rates (r² = 0.86, p<0.003). Population differentiation for growth rates (Q_ST) was significantly higher at 22°C than population differentiation at neutral microsatellite loci (G_ST) consistent with local adaptation for growth at higher temperatures. We found that <i>R. commune</i> has a high potential to rapidly adapt its growth rate to different thermal environments as heritability estimates ranged from 0.58 to 0.77. Our results suggest that this globally distributed pathogen has adapted to local climatic conditions, not through a shift in temperature optimum, but rather by acquiring generally fast growth in cooler, more variable climates or slow growth in warmer, more constant climates. This latter finding implies that there may be costs associated with fast growth under warm, constant climates.<p><p>Keywords: Fungus, Cereals-Grains