The genetics of resistance to five races of downy mildew (Plasmopara halstedii) in sunflower (Helianthus annuus L.)

These studies were undertaken to determine whether downy mildew resistance genes in sunflower were independent as first reported, or linked as suggested by more recent hypotheses. The segregations for downy mildew reaction of 111 F-3 progenies from a cross between a susceptible line and a line with Pl2 were used to locate this gene on the sunflower consensus RFLP linkage map. It was shown that Pl2 was linked to the same RFLP markers on linkage group 1 as Pl1 and Pl6, mapped earlier, and at a very similar distance. The F-3 progenies showed exactly the same segregation patterns when tested with race 1 and race D. One hundred and fifty four progenies from a cross between a susceptible line and HA335, containing Pl6 (considered as giving resistance to all Plasmopara halstedii races), were tested with the five French downy mildew races, 1, A, B, C and D. Two progenies were observed to show segregation for races 1 and D, while appearing homozygous-resistant to races A, B and C. Tests on F-4 progenies confirmed this separation of resistances with fixation of susceptibility to races 1 and D and resistance to races A, B and C. It is concluded that the Pl6 gene is not a ''strong'' gene, giving resistance to all downy mildew races, but rather a cluster of genes, each providing resistance to one, or a few, downy mildew races. The genes giving resistance to races 1 and D, on one hand, and to races A, B and C, on the other hand, must be very closely linked, with about 0.6cM between the two groups.