. 2015. Timing of inoculation and Fusarium species affect the severity of Fusarium head blight on oat. Can. J. Plant Sci. 95: 517Á524. The influence of timing of inoculation and pathogenicity of four Fusarium spp. (F. culmorum, F. graminearum, F. sporotrichioides, and F. avenaceum) causing Fusarium head blight (FHB) were examined on 12 oat genotypes under controlled environmental conditions in two separate sets of experiments. In the first set, early inoculations with F. graminearum at or before the complete emergence of ears resulted in little or no visible FHB symptoms but deoxynivalenol (DON) contents ranging from 0.4 to 2.6 ppm were detected in the harvested grain. Severe levels of FHB were observed on these genotypes with infected spikelets ranging from 30 to 74% and DON concentrations, from 6.6 to 10.0 ppm, when plants were inoculated at or after the 50% anthesis stage. Inoculation at 50% anthesis was considered the most appropriate as it allowed sufficient time for disease development and assessment prior to the physiological maturity of the plants. In the second set of experiments, F. culmorum and F. graminearum were equally highly pathogenic, having areas under the disease progress curve of 45.3 and 47.3, respectively. Fusarium sporotrichioides was significantly less pathogenic than the two highly pathogenic species. The pathogenicity of F. avenaceum was intermediate and was not significantly different from those of either the highly pathogenic or the weakly pathogenic Fusarium species.