A comparison of the genetics of seedling and adult plant resistance to the spot form of net blotch (Pyrenophora teres f. maculata)

Spot form of net blotch (SFNB) (Pyrenophora teres f. maculata) is an economically damaging foliar disease of barley in many of the world's cereal-growing areas. The gene Rpt4 that confers seedling resistance to SFNB has been mapped on the long arm of chromosome 7H, but no genes for adult plant resistance (APR) have been identified. A lack of field resistance to SFNB in breeders' lines selected for Rpt4 led us to investigate the genetics of APR to this disease. Five doubled-haploid populations were phenotyped for seedling and adult plant reaction. Markers linked to Rpt4 explained a large part of the seedling variation, but little of the APR. In 2 mapped populations, major quantitative trait loci (QTLs) for APR distal to Rpt4 on chromosome 7H were identified. QTLs contributing to APR on chromosomes 4H or 5H were also identified in each population. Association of the 5H QTL with a gene for cereal cyst nematode resistance and the probable effect of this linkage on the historical development of cultivars with adult plant resistance to SFNB is discussed.