Agriculture Reference
In-Depth Information
15.2.5 Biotic factors affecting the monocycle
(a) Pathogen density
Besides the meteorological conditions, the amount of inoculum or the density
of lesions can influence the phases of the life cycle. The infection efficiency of
B. graminis
decreases drastically with higher inoculum density (Damgaard and
Østergård, 1996). Similarly, the germination rate of
P. striiformis
is reduced with
increasing urediniospore concentration, due to self-inhibiting substances produced
by germinating urediniospores (Macko
et al
., 1977).
For
B. graminis
, incubation period (Aust
et al
., 1978) and latent period
(Damgaard and Østergård, 1996) are negatively correlated with inoculum density.
Increasing infection density also shortens the latent period of
P. triticina
(Baart
et al
., 1991).
High lesion density was found to be detrimental to lesion size for
P. graminis
f.sp.
tritici
(Leonard, 1969) and
P. triticina
(Baart
et al
., 1991).
Spore production per mildew colony decreases exponentially as the number of
colonies per leaf increases (Rouse
et al
., 1984). For
B. graminis
f.sp.
tritici
, the
dependence of sporulation on lesion density was explained by the competition for
nutrients between lesions (Leonard, 1969). However, total production of
P. triticina
urediniospores per leaf seems to be relatively independent of lesion density, the
main limiting factor being the photosynthetic apparatus (Mehta and Zadoks, 1970).
The density of leaf rust lesions on wheat seedlings affects spore production
mostly by influencing lesion size. A high density of lesions results in smaller lesions
but the number of spores per unit of sporulating area remains approximately
constant at a given temperature on wheat seedlings (Robert
et al.,
2002) and adult
plants (Robert
et al.,
2004).
(b) Host resistance
Characteristics of the host plants can also change the monocyclic parameters due to
partial resistance, expressed after the seedling stage or at the adult plant stage.
For
B. graminis
, the monocyclic parameters such as infection efficiency (Royer
et al
., 1984), latent period (e.g. Asher and Thomas, 1984), colony number cm leaf
area (e.g. Heun, 1986), lesion size (e.g. Forche, 1981), sporulation intensity (e.g.
Dutzmann, 1985) and sporulation capacity (e.g. Royer
et al
., 1984) can differ
between cultivars at the same growth stage.
Similar effects have been detected for the rusts. For instance, the latent periods
observed for different wheat genotype and stripe rust race associations varied
between 11.0 and 14.8 days (Ghannadha
et al
., 1995).
Cultivars expressing partial resistance that results in a decrease of the rate of the
epidemic are called slow-mildewing or slow-rusting cultivars.
The susceptibility of cereal plants to powdery mildew or rusts can decrease
during plant development. The adult plant resistance is especially expressed on
leaves formed later in the season, i.e. on the flag leaf and on the flag-1 leaf. Adult
plant resistance can reduce the development of mildew and rusts in all phases of the
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