Assessing resistance to head blight in wheat cultivars inoculated with different Fusarium isolates

Four winter wheat cultivars, differing in resistance to Fusarium head blight (FHB), were spray inoculated under field conditions for three years with 18 isolates of two Fusarium species (F. graminearum and F. culmorum) obtained from different regions of the Czech Republic. Deoxynivalenol (DON) contamination, FHB severity (VSS), Fusarium damaged kernels (FDK) and reductions of thousand grain weight and grain weight per spike were measured to describe the aggressiveness level of isolates. Highly significant correlations were found between these traits, but correlation coefficients ranged from 0.45 to 0.99. Analyses of variance showed significant effects of year, cultivar, isolate and Fusarium species on all traits. Experimental year accounted for the highest proportion of variation (37-59%), followed by cultivar (11-39%), isolate (9-18%) and species (1-3%). Two-way and three-way interactions between the main factors were also statistically significant, but with generally lower contribution to total variation. Though the examined isolates of two Fusarium species differed largely in quantitative aggressiveness, they did not show any evident qualitative differences in virulence. Pathogenic specialization of isolates was relatively higher in FDK. This trait appeared to be less affected by the disease level than the content of DON, which strongly reflected environmental conditions, cultivar resistance and pathogenicity of an isolate. Under highly variable weather conditions the significance of differences between the cultivars was not often detected by a single isolate and it was necessary to look for isolate groups in order to get a higher percentage of significant differences and prove the moderate resistance of the cultivar Arina. The highest number of significant differences between the cultivars was reached after selection of isolates, the application of which resulted in pronounced cultivar effects (lower interaction with years) and which showed high correspondence with the average reaction of cultivars in different years (R2). Divergence in sensitivity to the environment could be another criterion for selecting sources of inoculum.