THE GENETICS OF RESISTANCE TO 5 RACES OF DOWNY MILDEW (PLASMOPARA-HALSTEDII) IN SUNFLOWER (HELIANTHUS-ANNUUS L.)

These studies were undertaken to determine whether downy mildew resist ance genes in sunflower were independent as first reported, or linked as suggested by more recent hypotheses. The segregations for downy mil dew reaction of 111 F-3 progenies from a cross between a susceptible l ine 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 s ame RFLP markers on linkage group 1 as Pl1 and Pl6, mapped earlier, an d at a very similar distance. The F-3 progenies showed exactly the sam e 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 Plasmopa ra halstedii races), were tested with the five French downy mildew rac es, 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 ra ces 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 cluste r of genes, each providing resistance to one, or a few, downy mildew r aces. The genes giving resistance to races 1 and D, on one hand, and t o races A, B and C, on the other hand, must be very closely linked, wi th about 0.6cM between the two groups.