Agriculture Reference
In-Depth Information
and fertiliser application to sowing or planting. These practices, which are designed
to optimise plant growth, also have an impact on soil microbial activity. They
therefore have a direct or indirect effect on soil-borne pathogens and may be
considered to be a means of managing plant health, in addition to plant growth.
The first objective of soil fertilisation is to satisfy crop nutrition demands.
Deficiencies in major and minor nutrients may affect plant physiology, increasing
infection levels and exacerbating yield losses caused by the disease. It may also have
direct effects on soil-borne pathogens and on the biological and physico-chemical
characteristics of the soil. The impact of nitrogen fertilisation on take-all of wheat
provides a good illustration of these complex interactions: the application of a
source of ammonium reduces take-all in most situations whereas nitrate applications
do not have the same effect (Huber
et al
., 1968). The uptake of NH
4
+
by roots
decreases the pH of the rhizosphere. Smiley and Cook (1973) suggested that
decreasing pH indirectly inhibits take-all by modifying the rhizosphere microflora at
pH values between 5 and 7, and directly below pH 5. Smiley (1978a,b) subsequently
reported that the application of a source of NH
4
+
increases the proportion of
rhizosphere pseudomonads antagonistic to
G. graminis
var.
tritici in vitro
to a
greater extent than the application of NO
3
. Sarniguet
et al.
(1992a) showed, in pot
bioassays and studies of fields cropped with take-all-infected winter wheat, that
applications of ammonium-based fertiliser made the soil less receptive to take-all
than applications of a nitrate or mixed (NH
4
NO
3
) fertiliser. The mixed fertiliser had
an intermediate effect. Sarniguet
et al
. (1992b) demonstrated that the frequency of
in vivo
antagonistic fluorescent pseudomonads was higher in the NH
4
+
-treated soil
than in the NO
3
-treated soil. This work also demonstrated that the presence of
rhizosphere pseudomonads can increase disease severity. These deleterious bacteria
were more frequent in the nitrate-treated than in the ammonium-treated soil. Thus,
antagonism observed
in situ
results from the overall effect of antagonistic and
deleterious microorganisms, and nitrogen fertilisation (the form of nitrogen applied)
affect these two biological components of soil receptivity to take-all.
Soil tillage affects soil structure, thereby affecting the behaviour of
microorganisms. It also affects the distribution of crop residues in the soil profile.
These residues remain in the top layers in no-tillage systems, but are buried by
ploughing. This factor is important if successful infection requires the presence of
infectious crop residues close to the soil surface, as is the case for eyespot on winter
wheat, disseminated by spores carried over short distances by wind and rain drops
(Colbach and Meynard, 1995). Soil tillage and other cultivation practices, including
sowing, may also disperse inoculum within the field and even between fields.
According to Truscott and Gilligan (2001), the observation that transmission
distances within existing patches are frequently smaller than the expansion of
patches between seasons suggests that there is a high level of mechanical inoculum
dispersal during harvest and cultivation.
Disease can only occur if susceptible crop plants are grown. Successful infection
from soil inoculum is more likely to occur with high inoculum and plant densities.
Disease propagation is then favoured by short distances between plants. However, in
the case of strictly soil-borne diseases, dispersal within a crop has been shown to
be very limited, and build-up of the disease to epidemic levels requires several
-
-
