Inheritance of gray leaf spot resistance in perennial ryegrass
Gray leaf spot, caused by Magnaporthe oryzae B. Couch, is a devastating disease of perennial ryegrass in United States. The objectives of this study were to (i) develop effective screening techniques for gray leaf spot resistance in perennial ryegrass; (ii) explore the genetic properties of the inheritance of gray leaf spot resistance to determine the correct breeding strategies; and (iii) investigate the genetic diversity of pathogen populations and the interactions between host genotype and pathogen isolates.
First, we developed two screening techniques (one growth chamber and one field inoculation) that successfully reproduced gray leaf spot symptoms in perennial ryegrass cultivars and consistently reflected their resistance level observed in natural epidemics.
Three diallel crosses were made in the springs of 2001, 2002 and 2003 with six, four and eight parents, respectively. Analyses of both growth chamber and field experiments indicated that additive gene effects are involved in gray leaf spot resistance. A small portion of dominant and epistatic effects were also detected.
The estimate of narrow-sense heritability for gray leaf spot resistance ranged from 0.57 to 0.87. The minimum number of effective genes was estimated between 2.1 and 4.4. The fairly high heritability values suggest that recurrent selection was an effective breeding program to improve resistance. Two cycles of genotypic recurrent selection significantly improved gray leaf spot resistance in subsequent generations.
To further investigate the host and pathogen interactions of gray leaf spot, eighteen isolates of P. oryzae from diverse geographical and host origins were classified using RAPID markers. Seven isolates chosen to represent the range in diversity of pathogen were used to inoculate eight perennial ryegrass genotypes. Analyses suggested the coexistence of both interaction and additivity in resistance. No specificity was detected.
In conclusion, host resistance is an ideal approach to control gray leaf spot. Improving resistance to this disease in perennial ryegrass is feasible and effective when using a genotypic recurrent breeding program.
0480: Plant pathology