Reduction in aggressiveness among hybrids between host-specific pathotypes of Magnaporthe oryzae is caused by reduced ability to overcome adult resistance at the level of penetration

A genetic cross between a Triticum isolate (pathogenic on wheat) and a Setaria isolate (pathogenic on foxtail millet) of Magnaporthe oryzae yielded several F₁ cultures that were virulent on both wheat and foxtail millet at the primary leaf stage. To estimate whether these cultures survive in nature, they were sprayed onto 1-, 2-, and 3-week-old wheat and foxtail millet. As the age of the inoculated plants increased, the lesion number and size were greatly reduced. The F₁ cultures were almost nonpathogenic on both wheat and foxtail millet at the 3-week-old stage. Cytological analysis revealed that the low pathogenicity of the F₁ cultures on older plants was primarily associated with a reduced ability to penetrate the cuticle. When placed on wounded leaf surfaces, the F₁ cultures produced large lesions on 4-week-old wheat and foxtail millet. These results indicate that hybridization between species-specific pathotypes results in a reduction of aggressiveness. We therefore suggest that, even if such hybrids were produced in nature, they might not survive in the natural environment.