Agriculture Reference
In-Depth Information
mechanisms (Baker & Cook, 1974): (a) inability of the pathogen to establish or persist,
(b) the pathogen establishes, but causes little or no damage, (c) the pathogen establishes
and causes disease, but the disease becomes less important, despite the presence of the
pathogen in the soil. Höper & Alabouvette (1996) distinguished between pathogen sup-
pression and disease suppression, the former referring to the ability of the soil to limit the
inoculum density of the pathogen and its saprophytic activity, and the latter to the capac-
ity of the soil to restrict disease development even though the host, quantity of pathogen
inoculum and the environment appear favourable. Therefore, the capacity of a soil for
disease suppression will be determined by its effects on the processes of colonisation
and infection by the pathogen, and the subsequent development of disease symptoms.
Importantly, pathogen suppression and disease suppression might not necessarily be cou-
pled, and some soils might be pathogen suppressive but not disease suppressive and vice
versa (Höper & Alabouvette, 1996).
It is thought that the suppressiveness of a soil is determined mainly by its microbial
properties, especially since the suppressive effect can be destroyed by sterilisation (Peters
et al.,
2003). These microbial properties include the presence of rhizosphere and root
endophytic bacteria, which disrupt pathogen infection by various means, including pro-
duction of antibiotics, siderophores, nutrient competition and induction of systemic resis-
tance (Peters
et al.,
2003; Sturz & Christie, 2003). Other possible mechanisms include
predation of fungal hyphae by soil microfauna and competition from arbuscular mycor-
rhizal fungi (Workneh & van Bruggen, 1994; Knudsen
et al.,
1995; Azcon-Aguilar &
Barea, 1996).
It has been suggested that given the effects of soil structure on soil micro-heterogeneity
and microbial activity, suppressiveness is likely to be dependent on soil structure (Ball
et al.,
2005). For example, a poorly structured soil will restrict the activity and movement
of soil organisms, with consequences for predator-prey relationships. This, in turn, will
have consequences on pathogen survival, the spread and survival of introduced microor-
ganisms (e.g. biocontrol agents) and infection by fungal pathogens (Rattray
et al
., 1993;
Young & Ritz, 2000; Otten
et al.,
2001).
van Bruggen
et al.
(2006) argue that healthy soils are more suppressive to soil-borne
plant pathogens than biologically impoverished soils. These authors defi ne a healthy
soil as a stable soil system with high levels of biological diversity and activity, inter-
nal nutrient cycling, and resilience to disturbance. The implication is that microbial
fl uctuations following a disturbance would dampen more quickly in a healthy than a
biologically impoverished soil (van Bruggen
et al.,
2006). The authors suggest that
regular addition of soil organic matter might increase background levels of microbial
activity, increase nutrient cycling, lower the concentrations of easily available nutrient
sources, increase microbial diversity and enhance natural disease suppression (van
Bruggen
et al.,
2006).
2.6
Intercropping
The simultaneous planting of more than one crop in the same area is called intercropping
and is an important feature of cropping systems in the tropics. Intercropping has been
reported to provide protection against pathogens in component crops (e.g. Boudreau &
Mundt, 1992; Fininsa, 1996), although effectiveness can vary depending on location and
