Agriculture Reference
In-Depth Information
Puccinia striiformis , which is endemic in northern Europe, the US Pacific
northwest, west Asia, China, South Africa and South America, has a microcyclic life
cycle with no known alternate host.
In the north-central and north-eastern United States and adjacent Canada, winter
survival of the rusts is rare; the primary inoculum usually originates from wheat in
the southern United States and arrives in the region in late spring, when
temperatures are too high for stripe rust epidemics (Line, 1995).
15.4 POPULATION DYNAMICS
15.4.1 Dynamics of virulences
Biotrophic pathogens are under selection pressure at all times to infect more hosts
and are notorious for their ability to adapt to newly-introduced resistances. Genetic
studies have demonstrated that race-specific resistance fits the gene-for-gene theory
for B. graminis , P. graminis f.sp. tritici and P. triticina. The patterns for resistance
genes and races of P. striiformis f.sp. tritici are also consistent with a gene-for-gene
system. Eighty-five different resistance/avirulence gene pairs have been defined for
the interaction between barley and B. graminis f.sp. hordei (Jørgensen, 1994; Caffier
et al., 1996a), more than 60 pairs between wheat and P. graminis f.sp. tritici, over
50 pairs for P. triticina and over 40 for P. striiformis (Johnson, 1992; McIntosh
et al. , 1995a; Line and Chen, 1996; McIntosh et al. , 2003).
On a regional basis, the composition of the fungal populations is largely
determined by the cultivars grown and their respective acreage. When new host
resistance genes are introduced, the population of B. graminis and Puccinia spp. can
adapt rapidly by selecting pathotypes with matching virulence gene combinations
(e.g. Andrivon and de Vallavieille-Pope, 1993; Bayles et al., 2000; Wan et al.,
2004). The changes in virulence frequencies can be caused by strong direct selection
as well as by indirect (hitch-hiking) selection (Hovmøller et al ., 1993; Huang et al .,
1995). The mean number of virulences per isolate can be rather stable (Andrivon
and de Vallavieille-Pope, 1995), but in Europe the tendency was towards an increase
in pathotype complexity over the years (Wolfe and McDermott, 1994). The
tendency for increase in race complexity in rust pathogens like P. triticina and
P. striiformis f.sp. tritici (Stubbs, 1985; de Vallavieille-Pope et al ., 1990; Line and
Qayoum, 1991; Bayles et al. 2000; Line, 2002), and for stability of complexity at an
intermediate level in powdery mildew fungi when several specific resistance genes
are introduced in the host population, is possibly related to the biology of the
pathogens, in particular, the presence of a sexual phase in the powdery mildew fungi
(Andrivon and de Vallavieille-Pope, 1995). Diversity is greater in sexually
reproducing populations of P. graminis f.sp. tritici (Groth and Roelfs, 1982) than in
asexual populations where a single pathotype often predominates.
Races with the highest complexity are found in clonally reproducing pathogens
long faced with race-specific host resistance genes, such as P. graminis f.sp. tritici
in the United States or P. striiformis f.sp . tritici in Europe. Conversely, fungi that
may undergo sexual reproduction at frequent intervals, such as B. graminis and
Puccinia coronata f.sp. avenae , or that have been only recently combated by means
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