EFFECTS OF BARLEY GENOTYPE ON INDUCED RESISTANCE TO POWDERY MILDEW

Infection of barley by an avirulent isolate of Erysiphe graminis f.sp. hordei reduces the development of powdery mildew disease by a virulent isolate, inoculated subsequently. This phenomenon, known as induced resistance, was studied in three sets of near-isogenic lines of barley, each of which included varieties with four different race-specific resistance genes. The extent of induced resistance, expressed as a reduction in the number of colonies formed, differed in lines carrying different race-specific resistance genes, being most effective in lines with Mla7, followed by Mlh, then Mla6 and Mla13. These differences may be related to the extent of the hypersensitive response to infection by avirulent spores. A further effect of induced resistance in reducing sporulation, taking the number of colonies into account, was least effective in Mlh lines, with the other lines ranking as they did when induced resistance was expressed as colony numbers. By contrast, the genetic backgrounds of the three near-isogenic sets had similar effects on the fractional reduction of colony number by induced resistance. The effect of induced resistance on sporulation was strongest in the most resistant background, and weakest in the most susceptible.