© Benaki Phytopathological Institute
Sakr
2
major genes giving race specific resistance
to downy mildew have been mapped since
1995. So far, there appear to be two main
clusters, one on LG8 (and a second on LG13
(Radwan
et al
., 2002). Until the present day a
number of
Pl
genes have been reported (
Pl1-
15
,
Plv
,
Plw
,
Plx-z
,
Mw
,
Mx
,
Plarg, PlHA-R4
), and
position of 11 genes has been determined
on the SSR genetic map. It is usually the case
that new resistance genes provide resis-
tance to earlier races of the pathogen. The
second is quantitative resistance which is
controlled by minor genes (QTLs) and tends
to impact the rate of disease development
(rate reducing) rather than producing a dis-
ease-free plant (Tourvieille de Labrouhe
et
al.,
2008). Quantitative resistance is gener-
ally more stable and equally active against
many races of pathogen. The three major
QTLs were located on LG 1, 9 and 17, and ex-
plained 54.9% of the total phenotypic vari-
ance. Vear
et al.
(2008) reported two QTL
located on LG 10 and LG 8, and suggested
microsatellite markers ORS613 and ORS389
for their detection.
The interaction of avirulence genes of
P.
halstedii
and sunflower genotypes carrying
Pl
resistance genes led to the appearance
of new virulent isolates. Indeed, it has been
possible to identify up to 35 races, with dif-
ferent virulence patterns (Gulya, 2007).
Plas-
mopara halstedii
is characterized by a high
level of evolutionary potential expressed by
high virulence, aggressiveness and a great
potential in developing new races. Varia-
tion of virulence in
P. halstedii
has been the
object of several studies (Tourvieille de La-
brouhe
et al.,
2000, 2010; Sakr, 2011b, 2012c)
because several cases of major monogenic
resistance have been overcome rapidly by
the parasite. Downy mildew races have been
observed by Sakr (2009, 2011a,b,c, 2012a,c,
2013) and Sakr
et al.
(2009) to show consid-
erable differences in aggressiveness, which
affects host response. The most aggressive
races often produce symptoms which can-
not be compared with those produced by
the less aggressive ones. In this study, we
use terms according to the definitions of
Van der Plank (1968). Virulence is used to in-
dicate the qualitative component of patho-
genicity that is expressed vertically and ex-
pressed as specific disease-causing abilities.
Aggressiveness is used to indicate the quan-
titative component of pathogenicity that is
expressed horizontally irrespective of plant
cultivars or species and expressed as non-
specific disease-causing abilities. Pathoge-
nicity is used as a general term indicating
an ability to cause disease symptoms. To
explain the two components of
pathoge-
nicity
, the most destructive diseases of the
cultivated potato caused by
Phytophthora
infestans
could be elaborated. There are two
identified populations: old populations (e.g.
races US1, US6 and US7) and new ones (e.g.
races US8 and US11). Miller
et al.
(1997) found
that new races were more virulent and ag-
gressive than old ones.
In this review, data concerning patho-
genic variation in sunflower downy mildew
populations are briefly presented, since
knowledge of pathogenicity in
P. halstedii
would help to understand the dynamics of
pathogen populations that use their viru-
lence and aggressiveness to better improve
adaptation to their environment. Moreover,
the following questions are addressed: What
are the characteristics for the two compo-
nents of pathogenicity: virulence and ag-
gressiveness? How does the pathogen
evolve its pathogenicity under resistance
selection pressure? What are the relation-
ships among several morphological, genetic
and pathogenic traits? Is there an interven-
tion between pathogenic variation and du-
rable resistance in sunflower, at least from
the point of view of a plant pathologist and
epidemiologist? Such studies are needed
because sunflower downy mildew was con-
trolled until mid 2000 by using vertical re-
sistance which led to the appearance of new
P. halstedii
races (Tourvieille de Labrouhe
et
al.,
2000; Viranyi and Spring, 2011).
Plasmopara halstedii race evolution in
sunflower cultivated zones and molecu-
lar characterization of the pathogen
This species is widespread in all sunflow-
er-growing countries with the exception
1,2,3 5,6,7,8,9,10,11,12,13,14,...34