Fusarium head blight (FHB) is a widespread, destructive disease of wheat and barley that is primarily caused by Fusarium graminearum Schwabe. F. graminearum produces mycotoxins such as deoxynivalenol (DON) and nivalenol (NIV) in infected grain. Fungicide application is the most practical way to control FHB, but it is not consistently effective. To obtain better chemical control, the timing of application is important. In both wheat and barley, FHB fungicides have been usually applied at or near

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... anthesis (fl owering stage) regardless of the cultivar because this stage in wheat is most susceptible. However, we found that in barley the most critical time for F. graminearum infection and mycotoxin accumulation differs among cultivars. Whereas chasmogamous (open-flowering) cultivars were most susceptible at anthesis, cleistogamous (closed-fl owering) cultivars showed good resistance at anthesis, but were relatively susceptible after spent anther extrusion (SAE). As expected from these observations, field experiments using thiophanate-methyl fungicide indicated that the optimal timing for chemical control in cleistogamous barley is around the beginning of SAE, rather than at anthesis. For wheat, the manner in which DON and NIV accumulate in grain infected with F. graminearum and the infl uence of the time of infection on mycotoxin accumulation were investigated. High levels of DON and NIV were produced in grain beyond days after anthesis. This indicates the importance of the late stage of grain development in mycotoxin contamination in wheat, suggesting that control strategies that cover both the early and late stages of grain development should be developed.