Nature of wheat resistance to Fusarium head blight and the role of deoxynivalenol for breeding

Twenty (1990-93) and 25 (1994-96) wheat genotypes with different degrees of resistance and origins were tested with seven and eight isolates, respecti vely, of Fusarium graminearum and four Fusarium culmorum isolates of divers e origin in Europe. Infection severity depended largely on the genotypes an d the isolates used. Head blight values, yield response and kernel infectio n values revealed close but varying relationships with deoxynivalenol (DON) content. This variability is explained by the presence of tolerance mechan isms which affect the relationship between Fusarium head blight severity an d yield response. Kernel infection resistance accounted for decreasing Fusa rium head blight values. Genotypes were found with lower infection severity and higher DON contamination and vice versa. Evidently, the cultivar has a significant influence on DON production in the infected tissue, i.e. highl y susceptible genotypes may have moderate or low accumulation of DON. Howev er, in the most resistant genotypes showing no infection to any of the isol ates or only sporadic symptom development, no or very low accumulation of D ON was detected. Resistant genotypes gave a stable reaction with b-values c lose to zero for all trails tested. Susceptible genotypes were unstable und er different epidemic conditions acid their stability was different for the traits investigated. Therefore, the mean of b-values is suggested to bette r describe the stability of the wheat genotypes. Significant positive relationships were found between aggressiveness of the isolates and their production of DON in the infected grain. The correlatio n improved significantly for the nivalenol-producing isolate (F89.4 from Fr ance) when the sum of DON and nivalenol contents were considered. This indi cates that the total trichothecene toxin-producing capacity of the isolates may be a decisive component of pathogenicity. Since the tests included isolates from different European countries the res ults provide further proof that no host specificity exists within these pat hogens in Europe. This was also valid for kernel infection, yield response and DON accumulation. Therefore, the nature of resistance is horizontal. Th e results also support the view that there is no difference between the res istance of the host plant to F. graminearum and to F. culmorum.