Aggressiveness of Fusarium graminearum sensu stricto Isolates in Wheat Kernels in Argentina

This study was designed to investigate the degree of aggressiveness of Fusarium graminearum sensu stricto isolates and its relationship with trichothecene production. In order to characterize Fusarium strains aggressiveness, disease severity was visually assessed as the percentage of spikelets bleached per ear. The severity ranged from a minimum of 27.19% seven days after inoculation to a maximum of 84.73% at the end of the experiment. At maturity the ears were harvested and threshed for grain weight determination. All treatments showed significant differences in kernel weight with respect to the control plants, with a yield reduction of 35-85% in comparison with the yield of the control. Grains infected by F. graminearum may contain significant levels of mycotoxins like trichothecenes. No correlation was found between aggressiveness and the toxins in the kernels at maturity, taking into consideration not only the quantity but also the type of toxin (nivalenol, deoxynivalenol or their acetylderivatives). Kernel weight reduction was a better estimator of the presence of deoxynivalenol in the kernels than the area under the disease progress curve (AUDPC) calculated with severity ratings. The amplified fragment length polymorphism (AFLP) technique was used to establish genetic relationships between 18 Argentinean isolates and eight reference strains of the Fusarium graminearum complex. All the isolates studied grouped with the two F. graminearum s. str. reference isolates, with a similarity coefficient greater than 75%. The other reference strains of the F. graminearum complex were clearly separated, with similarities ranging between 55 and 73%. The AFLP groups had no relationship with toxin accumulation on kernels or with the geographical origin of the isolates. Great heterogeneity was found in the AUDPC, yield reduction and toxin accumulation values across the regions.