Fusarium head blight (FHB) is a devastating cereal crop disease that can cause significant yield losses and contaminate grain with hazardous mycotoxins. Recent data suggest that elevated carbon dioxide (CO₂) can increase wheat susceptibility to FHB and mycotoxin contamination. Thus, resilient host genetic resistance to Fusarium is key to ensuring the safety and security of our grain supply. There are different types of host resistance mechanisms. Resistance to initial infection (type I) and resistance to fungal spread between florets (type II). Type I resistance is thought to provide protection against fungal infection through plant morphological traits including height and anther extrusion. The Fhb5 qualitative trait loci (QTL) is associated with both type I FHB resistance and anther extrusion. Since elevated CO₂ has been shown to affect wheat floral development, we hypothesized that elevated CO₂ may compromise Fhb5 associated resistance to initial infection. Four wheat varieties which contain different combinations of the Fhb1 (predominant marker for type II resistance) and Fhb5 QTL: Alsen (Fhb1 and Fhb5), Bolles (Fhb5), Rollag (Fhb1), and Glenn (neither QTL), were evaluated for differences in initial infection at 400 ppm and 1000 ppm CO₂. Interestingly, while there was an on average increase in disease for all four varieties evaluated, only Alsen and Bolles which contain FHB5 had a significant increase in disease at elevated CO₂. The amount of deoxynivalenol mycotoxin contamination was significantly higher at elevated CO₂ for all four varieties, but the significance of this difference was much greater for Alsen and Bolles. Experiments assessing differences in variety anther extrusion are currently underway.