Genetic analysis of crown rust (Puccinia coronata f. sp. lolii) resistance in a perennial ryegrass (Lolium perenne) genotype

In order to study the inheritance of resistance to crown rust (Puccinia coronata f. sp. lolii) in perennial ryegrass (Lolium perenne), we crossed a resistant, diploid genotype (A6128/05) with a susceptible genotype of the cultivar Aurora to generate a F1 population. Individuals of the F1 population were screened for their response to three single-pustule isolates (SPI) of crown rust in a detached-leaf segment test under growth chamber conditions. Out of the F1 population, we selected two genotypes (I and II) based on the reaction to SPI of crown rust: genotype I was resistant to SPI A, D and F. Genotype II was resistant to SPI A, but susceptible to SPI D and F. Both genotypes were backcrossed with a second, susceptible Aurora genotype to generate two F2 populations. A resistant genotype out of each of the two F2 populations was used to generate a F3 population by backcrossing with a susceptible Aurora genotype. In the F2 and F3 populations, derived from a cross with the genotype I, a good fit to a segregation ratio of 1R:1S (resistant:susceptible) was observed for the three SPI A, D and F. The frequency of resistant and susceptible plants within the F2 and F3 populations, derived from the genotype II, resulted in a ratio of 1R:1S for SPI A. In contrast, the F2 and F3 progeny was susceptible to SPI D and F. These results provide evidence that at least two different dominant genes for crown rust resistance occur in the resistance donor A6128/05.