Agriculture Reference
In-Depth Information
an important source of inoculum for subsequent crops. Inoculum from infected crops
in adjacent fields, or from longer distances - carried in air currents or rain generated
aerosols, in vectors or on passive carriers - is a major source of in-season infection.
It is the latter source that is generally the concern of the disease forecaster.
It is important to recognise that sole reliance on weather for forecasting must
assume that inoculum is always present. The absence of inoculum, or delays in its
initial arrival, will affect the reliability of forecasting systems dependent solely on
weather. Van der Plank (1963) covers in detail the spread of disease, the influence of
inoculum and cultivar resistance and fungicides in his seminal topic on disease
epidemics.
9.2.3 Host
The host, in terms of its resistance and the nature of that resistance, whether it is
absent, partial or total, will affect the speed of development of the disease and
therefore will interact with any forecasting scheme.
The habit of the host, for example the structure of the crop canopy, will influence
the way disease moves about the plant and the architecture of the plant will
influence the microclimate and also the amount of damage caused by the effects of
wind - important where damage or wounding can aid pathogen entry.
9.3 POLYCYCLIC AND MONOCYCLIC DISEASES
Plant pathogens can be split into two groups, those where there are many
generations in a season (polycyclic) and those where there is time to complete only
one cycle from infection to spore production and re-infection of the host
(monocyclic). For a disease that is capable of rapidly re-cycling, e.g. where the time
from infection to spore production can be a matter of a few days under favourable
conditions, such as potato blight (caused by
Phytophthora infestans
) and barley
powdery mildew (caused by
Blumeria graminis
f.sp
. hordei
), information may also
be required on the appropriate intervals between subsequent spray applications.
With monocyclic diseases, where the disease only has one infection chance
throughout the season, as the pathogen is slow to develop and will not produce a
succession of spores capable of reinfecting and causing significant damage, a means
of establishing the presence and amount of inoculum may be all that is required, e.g.
Sclerotinia
stem rot of oilseed rape (caused by
Sclerotinia sclerotiorum
) and eyespot
of cereals (caused by
Oculimacula
spp
.
).
9.4 EQUIPMENT
Investigations into the aetiology and epidemiology of the pathogen usually require
the definition of the optimum conditions required for the infection process,
multiplication, survival and dispersal of the pathogen to occur. Much of this is to do
with defining optimum temperature and water requirements for spore germination
