Agriculture Reference
In-Depth Information
that minimise inputs. This knowledge is put to use by specialist crop advisers to help
them to select the least-cost options that maximise disease control and yield in the
field.
11.3.2 Modification of the crop environment
The rate of development of a disease epidemic is highly sensitive to environmental
conditions as they affect spore production and dispersal, the infection process and
the length of the latent period. However, pathogens have evolved to take advantage
of any favourable interlude in the microclimate. In some instances (e.g.
M
.
graminicola
)
the result has been the evolution of a well-adapted pathogen capable of
causing severe yield losses in conditions as diverse as the US mid-west, the
Mediterranean basin, Australasia and north-west Europe. Such adaptation renders it
almost impossible to identify simple sets of infection conditions which might be
exploited either in the field or in prediction systems (Shaw, 1991).
In protected horticulture, manipulation of the environment is more readily
achieved than under field conditions and the control of ventilation and temperature
has long been used as a means of delaying the build-up of diseases such as those
caused by the grey mould pathogen (
Botrytis cinerea
)
and the downy mildews
(Peronosporales).
Environmental control also plays an important part in controlling the diseases of
stored products. The management of potato stores to prevent the development of
tuber rots provides an example - and also illustrates the need for a good
understanding of the epidemiological requirements of the pathogens concerned.
Cooling to retard the development of bacterial soft rots, for example, can exacerbate
losses caused by gangrene (causal agent
Phoma exigua
f.
foveata
)
if it is not
preceded by a high temperature 'curing period'. Such a warm period facilitates the
healing of wounds through which, even at low temperatures, the
Phoma
pathogen
can advance into the flesh of the tubers.
Although actual modification of the above-ground environment is difficult under
field conditions it is not completely impossible. The epidemic development of many
pathogens (e.g.
O. yallundae)
is favoured by the high humidities maintained within
very thick crops, and reduction of seed rate and or increasing row width has long
been advocated as a means of delaying their development. Orientation of rows of
phaseolus beans to the direction of the prevailing wind has been advocated to
maximise airflow through the crops and the availability of sunlight to the plants to
reduce the incidence of sclerotinia (Haas and Bolwyn, 1972). Similarly, Steadman
(1983) cites the increased susceptibility of closed canopy cultivars as opposed to
open canopy types, which are more resistant.
Many splash-dispersed diseases rely on the ability of rain droplets to move freely
between individual plants and leaves within the canopy. In these situations, crop
density can have a significant influence on the epidemic development. One of the
best examples is septoria tritici blotch, for which disease the interaction with canopy
structure has been explored (Lovell
et al
., 1997)
.
The combination of proximity of
inoculum to susceptible leaves and the coincidence of rainfall able to disperse spores
