POTATO LATE BLIGHT - The Epidemiology of Plant Diseases - page 455

information to support management strategies comes from experiments done in
various environmental conditions and preferably in places where different P .
infestans genotypes are present. However, exhaustive evaluation of treatments by
classical experimental procedures (controlled conditions and field trials) is often of
low practical value. Furthermore, in many areas where pathogen populations are not
variable, it is not appropriate to introduce new genotypes, especially for field
experiments. However, as an inexpensive proxy for exhaustive field experiments,
assessment of treatments, including the effects of different genotypes and
environmental conditions, can be accomplished using late blight models. Such
analyses commonly require only computer time and desktop brainstorming time
when analyzing outputs.
17.4.1 Modelling
Potato late blight is a classic polycyclic disease (many generations per season) and
has been the subject of analysis via mathematical models. The goals have been to
explain phenomena or to investigate outcomes of certain conditions. Nevertheless,
the contribution of models extends beyond representation and analysis of epidemics.
Models are crucial for a systems analysis approach of evaluating management
practices and field experimentation must be conducted to test the predictions of
analysis.
Modelling of late blight has utilized both analytical and simulation approaches.
Analytical models can provide a general insight, whereas greater specificity is
possible from the use of computer simulation models. To date, all models have
assumed that the pathogen population is homogeneous and responds uniformly to
abiotic and biotic influences.
Analytical models can aid interpretation of temporal and spatial dynamics of late
blight epidemics. Progress models such as the exponential, logistic, and Gompertz
have been used to describe late blight epidemics and answer epidemiologically
relevant questions. Models can be used to compare the effects of diverse control
measures: environmental favourability (Gilligan, 1990), efficacy of resistant
genotypes (Ojiambo et al ., 2000), or resistance deployment (Andrivon et al ., 2003)
on late blight epidemics. On the spatial side, the spread of potato late blight through
'waves' has been investigated (Minogue and Fry, 1983b; Scherm, 1996). Recently,
using geographic information systems (GIS), the temporal and spatial patterns of
P . infestans genotypes were studied (Jaime-Garcia et al ., 2000). It is also possible to
assess the likelihood of occurrence of genotypes with important epidemiological
features such as fungicide resistance or mating type in an area (Jaime-Garcia et al .,
2001). This information can be useful for decision-making regarding, for instance,
the choice of proper fungicide to be applied and assessment of high-risk areas where
the likelihood of disease development is high and where more intensive fungicide
sprays should be considered. From the population genetics point of view, regions
containing both A1 and A2 mating types can increase the chances of sexual
reproduction; thus more attention should be devoted to late blight control in these
areas.