Comparative genetic analysis of quantitative traits in sunflower ( Helianthus annuus L.) 1. QTL involved in resistance to Sclerotinia sclerotiorum and Diaporthe helianthi.

Sclerotinia sclerotiorum and Diaporthe helianthi are important pathogens of sunflower ( Helianthus annuus L.). Two hundred and twenty F2-F3 families were developed from an intraspecific cross between two inbred sunflower lines XRQ and PSC8. Using this segregating population a genetic map of 19 linkage groups with 290 molecular markers covering 2,318 cM was constructed. Disease resistances were measured in field experiments during 3 years (1998, 1999 and 2000) for phomopsis and 2 years for S. sclerotiorum (1997 and 1999). QTL were detected using the interval mapping method at a LOD threshold of 3. A total of 15 QTL for each pathogen resistance were detected across several linkage groups, confirming the polygenic nature of the resistances. These QTL explained from 7 to 41% of the phenotypic variability. The QTL for phomopsis resistance, in the 3 years of tests, mapped in the same region, and this was also true for some forms of S. sclerotiorum resistance in the 2 years of tests. On linkage group 8, QTL affecting resistance to both S. sclerotiorum and D. helianthi mycelium extension on leaves colocalised, suggesting a common component in the mechanism of resistance for these two pathogens. The colocalisation of QTL and breeding for resistance to S. sclerotiorum and to D. helianthi by pyramiding QTL in sunflower are discussed.