Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 68

was stability until 1988 and 1989 when races 710 and 703 were reported
(Tourvieille de Labrouhe et al. 1991). It now appears that race 100 is extremely
homogeneous and probably homothallic and that races 710 and 703 are
more closely related to North American isolates (Roeckel-Drevet et al. 2003).
It was presumed that they were introduced. However, since then, other new
races have appeared specifically in France, in particular race 304, which is
now observed each year (Tourvieille de Labrouhe et al. 2000). Occasional
samples of several other races have been found (300, 307, 314, 700, 704 and
714; Penaud et al. 2003). From a geneticist's point of view, when their
virulence pattern is studied, they look like an F 2 between races 710 and 304.
Whether this is the case remains to be seen, but certainly there is now more
variability among European populations of Plasmopara halstedii, raising
questions as to the durability of major resistance genes.
Studies of resistance genes are made conventionally by crossing resistant
lines and then making a test cross of the F 1 hybrid with a susceptible line.
Mendelian segregation for resistance in this progeny then makes it possible
to define whether the lines carry the same (or closely linked) or independently
segregating genes. At least three clusters of genes have been found (a cluster
contains genes from different origins giving resistance to different races, but
which do not segregate independently) ( Fig. 2-6 ). The cluster including Pl1,
Pl2, Pl6 and Pl7 , has been shown to cover a large area, with about 0.5 cM
between resistance to races 100 and 300 on one hand and 700, 703 and 710
on the other, so that occasional segregants are susceptible to race 100 and

Figure 2-6 Efficiencies of Pl genes against Plasmopara halstedii races studied in France
(from Vear 2004). susceptible resistant.