Identification of a second linkage group carrying genes controlling resistance to downy mildew (Plasmopara halstedii) in sunflower (Helianthus annuus L.)

A sunflower line. XRQ, carrying the gene P15, which gives resistance to all French downy mildew races shows cotyledon-limited sporulation in seedling immersion tests, consequently, segregations in crosses with other downy mildew resistance sources were tested both by this method and by a secondary infection on leaves. P15 was found to segregate independently of P17 (HA338) but to be closely linked, or allelic, with P18 (RHA340). F-3 and F-4 progenies from a cross with a line containing Pl2 showed that P15 carries resistance to race 100 which segregates independently of Pl2. The Pl5 gene was mapped on linkage group 6 of the Cartisol RFLP map, linked to two RFLP markers, ten AFLP markers and the restorer gene Rfl. Tests with downy mildew race 330 distinguished P15 and P18, the first being susceptible, the second resistant, whereas both these genes were active against race 304 to which P16 (HA335) and Pl7 gave susceptibility. It is concluded that P15 and P18 are closely linked on linkage group 6 and form a separate resistance gene group from Pl6/Pl7 on linkage group 1. The origins of these groups of downy mildew resistance genes and their use in breeding are discussed.