Agriculture Reference
In-Depth Information
tillage uses a single cultivation, or even no cultivation (no-tillage, zero tillage, direct drill-
ing), and as a result leads to minimal soil disturbance. Minimum tillage and no-tillage
practices can be grouped together under the generic term conservation tillage (Sturz
et al.,
1997).
Tillage can bury pathogens deeper in the soil where they are less likely to become a
problem. It can alter soil texture, aeration, temperature, moisture and density, and can
also infl uence nutrient release in the soil with benefi ts to the crop (Ball
et al.,
2005).
Tillage also leads to clear fl uctuations in microbial activity and biomass in the soil (van
Bruggen
et al.,
2006). Reduced tillage or no-tillage is often associated with higher micro-
bial biomass and activity in upper soil layers compared to regular tillage (ploughing)
(van Diepeningen
et al.,
2005). This concentration of crop debris in the top layers of
the soil can promote the over-wintering and survival of numerous pathogens and has
prompted concern that increased disease and decreased yields will be the inevitable result
of using conservation tillage practices. Although this has proved to be the case under some
conditions, there have also been reports of decreases in the incidence of soil-borne dis-
eases (Sturz
et al.,
1997). As suggested by Sturz
et al.
(1997), such contradictory reports
may refl ect differences in root development and soil microbial biomass and activity under
different regimes. Thus, conservation tillage practices can lead to pathogen inoculum
concentrations several orders of magnitude greater than those found under conventional
tillage (Khan, 1975; McFadden & Sutton, 1975)
and, as a result, plant roots growing in
the upper soil layers might be more prone to pathogen infection (Sturz
et al.,
1997). In
contrast however, increased microbial biomass and activity in the top soil layers can give
rise to greater root density and root activity (Lynch & Panting, 1980; Carter & Rennie,
1984), which may offset the damaging effects of disease on yield, and might also provide
a highly competitive soil environment with resulting disease-suppressive effects (Chen
et al.,
1988).
In the United States, in the 1990s, losses of wheat and barley as a result of infection
by
Fusarium graminearum
(the cause of ear blight, head blight or scab) were nearly
$3 billion (Windels, 2000). These losses were blamed, in part, on the use of conserva-
tion tillage, allowing pathogen inoculum to survive on crop residues, although the evi-
dence for increased disease severity under minimum tillage has not always been clear
cut (Bateman
et al.,
2007). Thus, minimum tillage was identifi ed as a risk factor for
F. graminearum
infection in wheat in mid-western USA, if the preceding crop was wheat
or maize (Dill-Macky & Jones, 2000). In Germany, the risk of
F. graminearum
infection
in wheat was not clear cut following no tillage, if maize, and not wheat, was the previous
crop (Yi
et al.,
2001). In the United Kingdom, there is evidence that minimum tillage and
maize cropping increase the risk of infection of wheat ears by
F. graminearum,
although
the risk depends on the effects of weather conditions on, for example, infection and inocu-
lum accumulation (Bateman
et al.,
2007).
In some recent work, severity of tan spot in wheat was found to increase under no-
tillage conditions, but was reduced following reduced tillage (Carignano
et al.,
2008). To
control blackleg (
Leptosphaeria maculans
) on canola (oilseed rape), it is recommended
that crop debris is buried in the autumn and a non-host crop be direct seeded the follow-
ing spring to avoid re-exposing the buried residue (Gladders & Musa, 1980; Kolte, 1985).
Recent research suggests that inoculum production by
L. maculans
decreased with increas-
ing duration of stubble burial in the fi eld over 10 months, before stopping completely
