Agriculture Reference
In-Depth Information
of the soil-borne phase of ergot (caused by
Claviceps purpurea
), for example, relies
on ploughing to bury the sclerotia to a depth of at least 50 mm to prevent the clavae
from reaching the surface when germination occurs. However, cultivations have
varying effects on crop pathogens and there are often complex interactions.
Delaying crop establishment will allow more time for the inoculum of the
pathogen to be destroyed by the activity of other soil microorganisms; but the most
effective method for controlling most soilborne diseases is sound rotational practice.
A few examples are given here to illustrate the often complex effects of rotation and
other husbandry practices on soil-borne pathogens.
(a) Bunt
Soilborne inoculum of
Tilletia tritici
is readily controlled by rotation because the host
range of the pathogen is limited to a single species (wheat) and it has limited
saprophytic ability. The soilborne inoculum does not produce airborne spores, nor does
the fungus produce long-lived resting bodies. However, recent work suggests that its
spores can remain viable for much longer than was previously thought possible
(Johnsson, 1990) and may be more important as an inoculum source than was
previously believed to be the case. Delayed sowing allows inoculum levels in the soil
to decline before the seeds germinate, but the benefits of this are partially offset by the
fact that in the colder conditions to which late sown winter wheat plants are subjected,
it takes the seedlings longer to grow through the coleoptile stage during which they are
most susceptible to infection. The initial level of inoculum is by far the most important
factor in the development of a bunt epidemic, but even with this disease other factors
must be taken into consideration when devising a control strategy.
(b) Take-all
Gaeumannomyces graminis,
the cause of take-all of cereals (see also Chapter 14),
has a wider host range and a greater ability to survive in the absence of a host than
has
T. tritici.
It is, however, restricted to the
Gramineae
and its saprophytic ability is
so limited that it may usually be controlled by a one-year break from cereals. The
saprophytic survival of
G. graminis
is enhanced by the ready availability of nitrogen
and any treatment that reduces nitrogen levels during the break year may be
expected to enhance its effectiveness in reducing inoculum levels. In a trial at ADAS
Drayton, UK, for example, a sequence of wheat crops was interrupted by a set-aside
break. In the following wheat crop take-all levels were lower where the set-aside
land had been summer cropped with
Phacelia
sp. or more particularly, with white
mustard than where it had been summer fallowed (ADAS unpublished data). Uptake
of nitrogen by the mustard and
Phacelia
may have helped to 'starve out' the
pathogen - an effect of 'catch crops' long ago noted by Garrett and Buddin (1947).
In the case of the mustard the effect is likely to have been enhanced by the
fungitoxic isothiocyanates released by the roots of the mustard plants and acting as a
natural biofumigant (see 11.2.3. below). When diseases are controlled by rotational
practice, any treatment which can accelerate the decline in inoculum levels during
