© Benaki Phytopathological Institute
Evolution in
P. halstedii
5
cluding the more aggressive isolates of rac-
es 100 and 3xx, and another group includ-
ing less aggressive isolates of races 7xx. It
is possible that variability between
P. hal-
stedii
races is due to the origin of pathogen.
Additional virulence genes may be found
in
P. halstedii
races as observed for another
oomycete
Phytophthora infestans
(Montarry
et al
., 2010).
E
volution of two components of patho-
genicity (virulence and aggressiveness)
in Plasmopara halstedii under resistance
selection pressure
Selective effect on pathogenicity due
to host resistance is an important aspect of
plant-pathogen interactions, which can be
divided into two parts: virulence and ag-
gressiveness. In plant-pathogen interac-
tions, the selection pressure exercised by
qualitative resistance on parasitic popula-
tions might lead to the appearance of new
virulent isolates. On the other hand, the
wide usage of host plant varieties present-
ing high levels of quantitative resistance
led to increases in aggressiveness (Stuke-
nbrock and McDonald, 2008). Cowger and
Mundt (2002) showed that wheat cultivars
with good quantitative resistance selected
more aggressive isolates of
Mycosphaerella
graminicola
. However, this is not always true,
Sullivan
et al.
(2005) reported that tobacco
cultivars with high levels of quantitative re-
sistance did not select for more aggressive
isolates of
Phytophthora parasitica
var.
nico-
tiana
e. Also, Flier
et al.
(2007) showed that,
following large-scale introduction of more
resistant potato varieties in organic produc-
tion systems in Europe, there was no shift
towards increased levels of aggressiveness
of
Phytophthora infestans
populations.
Concerning the influence of
Pl
genes se-
lection pressure on virulence of sunflower
downy mildew, the interaction of
Avr
aviru-
lence genes of
P. halstedii
and sunflower gen-
otypes carrying
Pl
effective genes led to the
emergence of new virulent pathogen iso-
lates in the field.
Plasmopara halstedii
moni-
toring revealed that between 1987 and 2008
there was a rapid increase in the number of
gressiveness of parasite and the level of host
quantitative resistance. It also reflects the
capacity of the fungus to invade host tissues
during the incubation period. Dwarfing is a
symptomcharacteristic of plants systemical-
ly infected by
P. halstedii
and is explained by
a decrease in the concentration of growth
hormone (Iodole Acetic Acid) in infected
tissue (Cohen and Sackston, 1974). This de-
crease in size can be observed at a very ear-
ly stage. Also, the index of aggressiveness
of the
P. halstedii
isolate was used to sum-
marize all values for two criteria (pathogen
virulence and aggressiveness) on sunflow-
er inbred lines in one value to facilitate the
comparison between the different
P. halste-
dii
races. The index of aggressiveness of
P.
halstedii
single zoosporangium isolate was
calculated as the ratio of (percentage infec-
tion × sporulation density) / (latent period ×
dwarfing).
Research to analyze aggressiveness has
been carried out in several sunflower downy
mildew races. Differences in aggressiveness
of
P. halstedii
races are indicatedwhenpatho-
gen isolates vary in the amount of damage
they cause on sunflower plants. Sakr
et al.
(2009) used two aggressiveness criteria, la-
tent period and sporulation density, to dif-
ferentiate between two races 100 and 710,
and it was found that race 100 was more ag-
gressive than race 710. Moreover, Sakr (2009,
2011a,b,c, 2012c, 2013) included four criteria
to distinguish among seven pathogen rac-
es. It was found that pathotypes 100, 300,
304 and 314 were characterized with short-
er latent period and higher sporulation den-
sity than pathotypes 710, 704 and 714 (Fig-
ure 1). Also, it was noted that all pathotypes
showed high percentage infection values
and caused a large reduction in seedling size
except for pathotype 314 involved in dwarf-
ing (Figure 1). Then, Sakr (2009, 2011a,b,c,
2012c, 2013) concluded that races 100 and
3xx were more aggressive than races 7xx
(Figure 1). Regarding virulence and aggres-
siveness reaction of pathogen races to sun-
flower inbred lines carrying several levels
of resistance, Sakr (2009, 2011a,b,c, 2012c,
2013) revealed two groups. One group in-
1,2,3,4,5,6 8,9,10,11,12,13,14,15,16,17,...34