Agriculture Reference
In-Depth Information
Determination of the infectivity from petal infection is a further refinement and was
commonly practised in Canada (Morrall and Thomson, 1995). Growers sampled
petals from their crops and incubated them on agar plates. After 3-4 days incubation,
any resultant fungal colonies were identified using high quality colour photographs
of colonies and also compared with a pure culture of
Sclerotinia
grown at the same
time. The percentage petal infection was used to assign a risk value to the crop. The
results must be ready in 3-4 days so that spray decisions can be taken. This test was
tried in England with limited success (Davies, 1995). The failures relate to the time
taken to identify
Sclerotinia
from colony formations. Many of the petals are
contaminated by
Botrytis cinerea
and it takes from 8-10 days for sclerotia to develop
which can distinguish the colonies. This was considered to be an unacceptable delay.
This highlights the requirement for immunodiagnostics/PCR tests specific to
Sclerotinia
to give more rapid identification.
9.9 CONCLUSIONS
Forecasting schemes, when successful, should provide the farmer with the information
necessary to provide a cost-effective means of protecting crops from the ravages of
disease. However, there are a number of factors which impinge on the success of
forecasting schemes.
9.9.1 The models
The models are all trying to interpret the biology of the pathogen in the context of
conditions which affect its development, survival and ability to infect and colonise
the relevant host. There is an infinite variety of possible combinations. Fortunately,
the advent of powerful computers has solved many of the technical constraints.
However, biological processes are in a constant state of flux, particularly with the
introduction of new strains and mating types, and therefore there is the potential for
more rapid changes in aggressiveness and temperature adaptations; it is unlikely that
all eventualities will be covered by even the most complex of models. This is an
important constraint, for if the models are too complex they may be impractical,
particularly if they are dependent on information being supplied by farmers or their
consultants.
Most of the models impose a cut-off. This is not necessarily arbitrary but one
that may be derived experimentally. However, it is likely to be a mean from a range
of results. Fungi are not aware of means. What, for example, is special about 15.7°C
but not 15.6°C (Polley and Smith, 1973), or 10°C and not 9.9°C, 90% RH and not
89.5%, 11 h and not 10 h 59 min (Smith, 1956)? This is clearly nonsense. It is now
possible to collect data by the minute rather than every 6 h from the synoptic
network. However, this gives a false sense of precision and can be likened to
blowing up a photograph from a newspaper - the dots that make up the picture are
seen in more detail but the subject loses focus and content. Using accumulated day
degrees (Croxall, 1978) gives a feeling of greater confidence, as it takes account of
seasonal variation, but why accumulate over 6°C and not 5°C?
These points were
