Agriculture Reference
In-Depth Information
ploughing than after direct drilling, but numerous studies have shown that the
opposite is more usually the case (e.g. Yarham and Norton, 1981). Various factors
are likely to be involved in this apparent anomaly: a thick layer of straw on the soil
surface can impede the splash dispersal of the conidia; the more prostrate habit of
the usually more shallowly sown direct-drilled plants makes it less likely that
splashed spores will be funnelled down to the stem base; and the greater availability
of nitrogen (a result of higher rates of soil nitrification) in ploughed soils in the
autumn increases the susceptibility of the young plants to infection (another example
of excess nitrogen increasing disease risk; see below).
Observations in the UK have indicated that yield responses to fungicides applied
for eyespot control are lower in wheat crops following oilseed rape than where the
previous crop has been either a cereal or a non-cereal crop other than rape (Cook
and Thomas, 1990). Infected debris from the wheat crop prior to a break crop is
normally buried by ploughing and brought to the surface again immediately prior to
the following wheat crop. Rape, however, is frequently sown into unploughed cereal
stubble, so infected straws from the previous crop will be left on the soil surface
during the break from cereals and the fungus will have exhausted its capacity to
produce more spores (and, therefore, its inoculum potential) before the next cereal
crop is taken, thus reducing disease incidence in that crop.
Moreover, it has been suggested that the rape crop may itself contribute to the
partial sterilization of the soil in which it grows. Increases in yields of wheat
following rape, far greater than those observed after crops such as linseed or oats,
have been observed in Australia, the United States and Chile (e.g. Kirkegaard
et al.
,
1994; Kirkegaard, 1996). The evidence suggests that some form of biofumigation
may be responsible; phenyl ethyl glucosinolate released from rape roots is converted
by microbial enzyme activity to isothiocyanates which are known to be toxic to
many pest and pathogen species.
Leaf debris from previous crops is more ephemeral than stem base debris but can
provide a direct inoculum source for many diseases, as for example when fallen
leaves affected by apple scab (caused
by
Venturia inaequalis
)
release ascospores
into the emerging leaf canopy during the spring (see also Chapter 18). Wind-blown
spores may cause infection when they are discharged from debris remote from the
crop, as in the case of septoria tritici blotch of wheat (caused by
Mycosphaerella
graminicola,
teleomorph of
Septoria tritici
)
.
This disease is notable for the uniform
infection that occurs in wheat crops wherever they are grown. Ascospore infection is
the most likely source of disease, straw debris providing a source of long-distance
ascospores (Scott
et al.,
1988; Shaw and Royle, 1989). Dissemination of
M
.
graminicola
inoculum as ascospores is widespread through the countryside so that
every susceptible crop might be expected to become infected sooner or later. Even
so, the presence of inoculum either within the field or in close proximity to it can
affect disease development. Studies by ADAS and Rothamsted Research, UK have
demonstrated that ploughing or straw burning can reduce the severity of infection, at
least in the early stages of an epidemic, and some work shows early gradients in the
incidence of
M.
graminicola
in crops adjacent to unploughed set-aside fields
(Yarham and Gladders, 1994). Later, the effects of proximity to within-field
or nearby inoculum will often be lost, as in the logarithmic stage of epidemic
