For many years, the gray leaf spot disease (GLS) caused by the fungus Cercospora zeae-maydis Tehon & Daniels, was not considered an important pathogen of maize (Zea mays, L.) in Brazil. However, the recent adoption of agronomical practices such as no-tillage and cultivation under central pivot irrigation systems increased the incidence and severity to the extent that GLS is now one of the most important diseases of maize. Isolates of C. zeae-maydis can be distinguished by two genetic groups (I and II) based on AFLP markers and on polymorphisms of the ITS and 5.8S rDNA regions. Until now, however, the biological implications of this distinction remain unclear. This study investigated whether isolates from the two genetic groups differ in aggressiveness towards maize. For this, symptoms of a susceptible hybrid were evaluated under greenhouse conditions with 9 and 11 isolates of C. zeae-maydis from groups I and II, respectively. Plants in the V3 growth stage were inoculated by placing sorghum seeds colonized with the pathogen in the leaf whorl and symptoms were evaluated with a visual rating scale 30 days later. On average, isolates of genetic group II were more aggressive than those of group I, with mean disease scores of 3.1 and 2.3, respectively. Differences were also observed between experiments, which suggested that group I and II might also differ in their fitness under different environments. This is the first report on differences in aggressiveness between the two genetic groups of C. zeae-maydis.
disease resistance; gray leaf spot; genetic variability; pathogenicity