Agriculture Reference
In-Depth Information
sufficient to classify an isolate's response one way or the other. A currently
important example is the response of many fungi to Q
o
inhibitor (QoI, also known as
strobilurin) fungicides (Sierotzki
et al.
, 2000a, 2000b; Chin
et al.
, 2001; Robinson
et al.
, 2002).
(b) Minimum inhibitory concentrations
Resistance to many other fungicides is quantitative in nature, so isolate responses
need to be measured by scoring these responses to several doses of the fungicide.
The most widely used measure of resistance is the median effective dose (ED
50
), that
which kills half the sample being tested. Another is the minimum inhibitory
concentration (MIC), the lowest dose sufficient to kill the pathogen.
The MIC might appear at first sight to be the easier of these two quantities to
estimate, as it is simply an empirical observation of the lowest dose that kills all
individuals. However, the estimated MIC is positively correlated with the
inoculation density. Suppose that a certain MIC is observed at a low inoculation
density: at a higher density of inoculation, there is a greater probability that the
inoculum will contain at least some individuals capable of surviving at the MIC
observed at the lower density. The MIC observed at the higher dose is therefore
likely to be higher than that at the lower dose.
(c) Effective doses
The ED
50
is estimated by fitting a curve of pathogen response (or survival) to the
dose series. An appropriate curve is generally that of probits or logits of pathogen
response against the logarithm of the dose. Such models can only be fitted
accurately if there are several doses covering the steep part of the dose-response
curve, some doses (including the zero dose - i.e. untreated material) to which the
pathogen does not respond at all and at least one dose which is completely effective
or nearly so (Brown, 1998).
The aim of applying a fungicide to a crop is to achieve near-complete disease
control. It is therefore often useful to estimate effective doses (EDs) higher than the
ED
50
such as the ED
95
, which kills 95% of the target pathogen. The value of
estimating higher EDs depends on the material being tested: they may be useful for
samples which are mixtures of genotypes, representing the whole target population,
but are all but meaningless for isolates which are genetically uniform. In the former
case, the difference between the ED
95
and the ED
50
reflects genetic variation in
responses because the greater the difference between the two EDs, the greater the
variance of response in the sample and therefore the greater the variation in the
population as a whole, if the sample is a good representation of the population. In
the latter case, however, all individuals of the fungus have the same genotype
(barring mutation) so any difference between the ED
95
and the ED
50
is merely the
result of environmental variation in the experiment.
If a sample contains many genotypes, the difference between the ED
50
and a
higher ED, such as ED
95
, may be of value in planning fungicide application regimes
