Agriculture Reference
In-Depth Information
of resistant cultivars, such as
P. coronata
f.sp.
avenae
in Canada (Chong, 1985,
1986), have less complex races and often a greater diversity of virulence
phenotypes. However, there is a large local effect of selection by resistance genes on
race diversity and complexity (Andrivon and de Vallavieille-Pope, 1995). In the US
south-central area, a new race has predominated since 2000, distinguishable by
molecular markers and by a narrow virulence spectrum from previous races in the
United States (Chen
et al.,
2002; Markell
et al.,
2004).
Diversity is higher in populations of cereal mildews than in populations of cereal
rusts.
Puccinia striiformis
f.sp.
tritici
is strictly clonal, evolving mainly through the
mutation and selection of the fittest clones (Steele
et al.
, 2001; Hovmøller
et al.,
2002; Roose-Amsaleg
et al.,
2002; Villaréal
et al
., 2002; Keipfer
et al.,
2003),
except in western Chinese populations, which are more diverse (Shan
et al.,
1998;
Enjalbert
et al.,
2002, 2004).
There are also cases of clonal populations of
B.
graminis
f.sp.
hordei
, as shown during the first years of cultivation of barley
cultivars carrying a new resistance gene, for example,
Mla7
and
Ml(La)
(Brown
et al
., 1993). Furthermore, a
B. graminis
f.sp.
hordei
clone of a race with a single
virulence dominated in epidemics on French winter barley cultivars carrying few
specific resistance genes to powdery mildew (Caffier
et al
., 1996b, 1999).
Selection
solely due to host resistance genes, without assuming any cost of virulence, might
lead to such results (Bousset
et al.,
2002).
A second factor that can change the composition of the fungal populations is the
migration of pathogens by wind. Limpert
et al.
(1999), for instance, assumed that
the observed increase in race complexity of
B
.
graminis
from western to eastern
Europe was due to spore transport in the main wind direction.
Similarly, new
P.
striiformis
races occurred in Denmark due to spore migration from England
(Hovmøller
et al.,
2002).
Within a field population, the proportion of races changes with time, depending
on the fitness of the pathotypes. Over a long period, the dynamics of
B. graminis
races changed cyclically over seasons with a high diversity of races after the
oversummering period of the pathogen due to recombination (Welz and Kranz,
1987) and migration and a decrease until the following summer due to selection
(Welz, 1988). Temperature seems to be an important factor that influences the
composition of the pathotypes (Eckhardt, 1987) by changing the fitness of races.
Furthermore
,
different races are selected in different zones of the world; for
instance, five different groups of
P. striiformis
races were found in Europe (Stubbs,
1988), a main division being between north-western Europe and southern and
eastern Europe, due to resistance gene deployment and possibly to climatic
conditions.
15.4.2 Dynamics of fungicide resistant subpopulations
Besides the pathotypes which are characterized by their virulence genes, sub-
populations within the mildew population can be defined with respect to fungicide
resistance. Powdery mildew isolates may differ in a wide range of sensitivity to
fungicides that can be characterized by the respective ED
50
value. The application of
