Agriculture Reference
In-Depth Information
time lags,
may
show complicated dynamics. Simple models of two interacting
populations in continuous time, without time lags, are often simple to construct and
may capture the essentials of host-pathogen interaction. Mathematically, however,
they can
only
converge to an equilibrium or to steady repeating oscillations. This can
be a trap, since apparently similar descriptions of a pathosystem may be capable of
showing very different behaviour and it may be difficult to distinguish the models
by comparison with the very short runs of data that are available.
7.8.2 Host-pathogen population linkages
In agricultural settings, the host population may be only indirectly linked to the
pathogen population. In the most extreme case the same crop cultivar (or one with
equivalent disease resistance) may be planted, regardless of the disease level. This
may occur because there is no economic alternative, as with black Sigatoka and
Sigatoka diseases of banana caused by
Mycosphaerella fijiensis
and
M. musicola
(Marin
et al.
, 2003), where both marketing and breeding constraints restrict the
available cultivars, or it may simply be that agronomic and market imperatives
override consideration of disease resistance in choice of a cultivar, as with the
growth of wheat susceptible to
M. graminicola
in the UK during the 1980s and
1990s (Bayles, 1991). In other cases, planting areas of cultivars that suffer severe
disease may decrease, as in the classic boom-bust cycles of gene deployment in
cereals (Wolfe, 1984).
In some natural habitats, by contrast, the success of reproduction by the host is
directly linked to that of the pathogen. For example,
Linum marginale
is a wild
relative of flax native to Australia. Plants
infected with
Melampsora lini
(flax rust)
do not perennate, whereas healthy plants may survive many years (Burdon and
Thompson, 1995). Likewise, Carlsson and Elmqvist (1992) showed that populations
of
Silene alba
infected with
Microbotryum violacearum
smut were smaller than
those which were uninfected. They also showed that this was at least in part because
the populations were seed limited and seed output was reduced by infection with the
smut.
To analyze long-term dynamics, it is necessary to study population changes over
the entire pathogen life-cycle, including if they exist, seasons where a new host is
unavailable for infection: often in the dry season(s) in tropical areas, typically winter
in temperate countries. The simplest example to examine is an annual crop plant,
affected by a disease that infects entire plants annually from an over-seasoning
source, then grows within plants and produces new propagules which will carry the
disease forward to the next season. A well-known example would be
Sclerotinia
sclerotiorum
infecting oilseed rape (canola) and a range of other crops. Wind-blown
ascospores initially colonise fallen petals, producing a mycelium which is then
capable of penetrating a leaf on which the petal may be lying. These leaf infections
become systemic and the pathogen eventually produces cankers in the stem
containing sclerotia which will survive in the soil until the following spring when
sclerotia near the soil surface produce apothecia from which ascospores are
discharged to infect the new season's crop. Thus, to understand population changes it
