Agriculture Reference
In-Depth Information
is to reduce the incidence of diseases, pests or weeds that are diffi cult to control with
pesticides and for this reason, short rotations of two to three crops are usually employed.
In the United States, for example, the majority of the maize crop is grown on a two to three
year rotation, while in the United Kingdom, barley and wheat usually form the main part
of the rotation, with breaks of oilseed rape, beans, peas or potatoes (Ball et al., 2005).
Continuous cropping with the same susceptible host plant will result in the establish-
ment of a soil population of pathogenic microbes. Crop rotation avoids this and is often
associated with a reduction in crop diseases caused by soil-borne pathogens (Janvier et al.,
2007). Using non-host or less susceptible crop plants for the rotation can lead to a decline
in the specifi c populations of plant pathogens in the soil and is best suited for biotro-
phs, since they require the presence of the specifi c living host for survival, or pathogens
with low saprophytic ability (Bailey & Duczek, 1996; Peters et al., 2003). Crop rotation
is less suitable for controlling root-inhabiting pathogens that survive saprophytically or
can exist for long periods in soil, for example, pathogens with tough survival structures
such as Rhizoctonia solani, Sclerotinia sclerotiorum and Pythium spp. (Sumner, 1982;
Umaerus et al., 1989). S. sclerotiorum is among the most non-specifi c and successful
plant pathogens and can attack a wide range of crops including oilseed rape (canola),
sunfl ower, fl ax, pea, bean, clover and potato (Morrall & Dueck, 1982). The number of
viable sclerotia of S. sclerotiorum in soil was unchanged after three consecutive barley
crops following canola (Williams & Stelfox, 1980), suggesting that the four-year rotation
between susceptible crops recommended to control sclerotinia stem rot might be inef-
fective (Kharbanda & Tewari, 1996). In contrast, rotation involving at least three years
between canola crops is usually associated with a reduction in severity of blackleg caused
by Leptosphaeria maculans (Petrie, 1986). Although L. maculans can survive in stubble
for more than fi ve years, its viability decreases with age and most damaging infections
arise from inoculum produced on two- to three-year-old stubble (Kharbanda & Tewari,
1996). Indeed, in South Australia, L. maculans was not detected in soils three or more
years after a canola crop (Sosnowski et al., 2006) and in this region a two-year rotation
between successive canola crops is encouraged (Gladders et al., 2006). With the barley
leaf scald pathogen Rhynchosporium secalis, the amount of primary inoculum available
for initiating epidemics can be decreased by rotation (Shipton et al., 1974), whereas short
rotations and reduced tillage which leave infected debris on the soil surface could lead to
severe epidemics of R. secalis in crops exposed to more primary inoculum (Zhan et al.,
2008).
2.3
The rate of pathogen spread within a crop can be reduced by altering the environment
within the crop, for example, by manipulating crop density, humidity and moisture
levels.
Reducing pathogen spread within the crop
2.3.1
Tillage
Tillage has indirect effects on pathogen spread and can also be used to reduce pathogen
inoculum in the soil. Conventional tillage uses primary and secondary cultivation to pre-
pare a seed-bed for planting and results in considerable soil disturbance, while reduced
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