Quantitative trait loci for resistance against powdery mildew in a segregating wheat x spelt population

Powdery mildew is one of the major diseases of wheat in regions with a mari time or semi-continental climate and can strongly affect grain yield. The a ttempt to control powdery mildew with major resistance genes (Pm genes) has not provided a durable resistance. Breeding for quantitative resistance to powdery mildew is more promising, but is difficult to select on a phenotyp ic basis. In this study, we mapped and characterised quantitative trait loc i (QTLs) for adult-plant powdery mildew resistance in a segregating populat ion of 226 recombinant inbred lines derived from the cross of the Swiss whe at variety Forno with the Swiss spelt variety Oberkulmer. Forno possibly co ntains the Pm5 gene and showed good adult-plant resistance in the field. Ob erkulmer does not have any known Pm gene and showed a moderate susceptible reaction. Powdery mildew resistance was assessed in field trials at two loc ations in 1995 and at three locations in 1996. The high heritability (h(2) = 0.97) for powdery mildew resistance suggests that the environmental influ ence did not affect the resistance phenotype to a great extent. QTL analysi s was based on a genetic map containing 182 loci with 23 linkage groups (24 69 cM). With the method of composite interval mapping 18 QTLs for powdery m ildew resistance were detected, explaining 77% of the phenotypic variance i n a simultaneous fit. Two QTLs with major effects were consistent over all five environments. One of them corresponds to the Pm5 locus derived from Fo rno on chromosome 7B. The other QTL on 5A, was derived from the spelt varie ty Oberkulmer and did not correspond to any known Pm gene. In addition, fiv e QTLs were consistent over three environments, and six QTLs over two envir onments. The QTL at the Pm5 locus showed a large effect, although virulent races for Pm5 were present in the mixture of isolates, Molecular markers li nked with QTLs for adult-plant resistance offer the possibility of simultan eous marker-assisted selection for major and minor genes.