© Benaki Phytopathological Institute
Sakr
10
unanswered. Virulence is a driving force in
host-pathogen co-evolution since it ena-
bles pathogen to overcome qualitative
R
re-
sistance genes. Aggressiveness enables the
pathogen to develop within the host plant
(Van der Plank, 1968). Indeed, the parame-
ters measuring changes on the level of vir-
ulence and aggressiveness (the two com-
ponents of pathogenicity) could be used to
evaluate the level of co-evolution of
P
.
hal-
stedii
with its sunflower host.
Furthermore, since it appears that
P. hal-
stedii
evolves new races through both muta-
tion and hybridization, this species may pro-
vide a useful model for further research into
the genetics of pathogen evolution. These
issues are not only important for inform-
ing disease management policy, but also
they address key questions in the genetics
of adaptation. Recently, few molecular stud-
ies have been performed in the
P. halste-
dii
–
H. annuus
system to explain resistance
from the plant’s side and virulence from the
pathogen’s side (Bouzidi
et al
., 2007; As-Sadi
et al
., 2011). Enriching the genomic resourc-
es available for exploring the interaction be-
tween
H. annuus
and
P. halstedii
is crucial for
research on
P. halstedii
, especially with re-
spect to discovering the effectors involved
in its pathogenicity. Finally, the pathosystem
of
P
.
halstedii
and
H
.
annuus
will continue to
evolve virulence in the pathogen unless it
was reengineered to make it less conduc-
tive to pathogen evolution (Sakr, 2011b) in
accordance with Stukenbrock and McDon-
ald’s (2008) hypothesis.
I gratefully thank Jalal Al-Attar for help in sta-
tistical analysis.
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