03-07-2013, 03:45 PM
Simulation of Potato Late Blight in the Andes. II: Validation of the LATEBLIGHT Model
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ABSTRACT
a mathematical model that simulates the effect of
weather, host growth and resistance, and fungicide use on asexual development
and growth of Phytophthora infestans on potato foliage, was
validated for the Andes of Peru. Validation was needed due to recent
modifications made to the model, and because the model had not been
formally tested outside of New York State. Prior to validation, procedures
to estimate the starting time of the epidemic, the amount of initial inoculum,
and leaf wetness duration were developed. Observed data for validation
were from field trials with three potato cultivars in the Peruvian
locations of Comas and Huancayo in the department of Junín, and
Oxapampa in the department of Pasco in 1999 and 2000 for a total of
epidemics. These data had not been used previously for estimating model
parameters. Observed and simulated epidemics were compared graphically
using disease progress curves and numerically using the area under
the disease progress curve in a confidence interval test, an equivalence
test, and an envelope of acceptance test. The level of agreement between
observed and simulated epidemics was high, and the model was found to
be valid according to subjective and objective performance criteria. The
approach of measuring fitness components of potato cultivars infected
with isolates of a certain clonal lineage of P. infestans under controlled
conditions and then using the experimental results as parameters of
LATEBLIGHT proved to be effective. Fungicide treatments were not
considered in this study.
INTRODUCTION
LATEBLIGHT is a mathematical model that simulates the
effect of weather, host growth and resistance, and fungicide use
on asexual development and growth of Phytophthora infestans on
potato foliage. Andrade-Piedra et al. (4) modified this model so
that it can be used in the Andes and, eventually, worldwide. This
was needed because the previous version of the model (referred to
as LB1990 [4]) is invalid outside the conditions for which it was
originally calibrated due to the following limitations: (i) some of
the original parameters and equations represent a P. infestans
population that has been displaced by new, more aggressive populations;
(ii) a key fitness component is assumed to be constant at
all temperatures, while experimental data shows the opposite; and
(iii) several parameters obtained by calibration are out of range of
experimentally observed values and, therefore, are biologically
meaningless (4). The modifications, therefore, included the incorporation
of improved equations for the effect of temperature on
lesion growth rate (LGR) (Table 1 provides a list of acronyms and
abbreviations) and sporulation rate (SR) in isolates from a clonal
lineage that belongs to the new populations of P. infestans; the incorporation
of temperature-dependent latent period (LP); and the
use of experimentally measured parameters of LGR, SR, and LP
of specific potato cultivars and pathogen lineages (4). The resulting
version of the model was referred to as LB2004 (4).
MATERIALS AND METHODS
Field experiments. Trials were conducted in three locations in
Peru: Comas and Huancayo in the department of Junín, and
Oxapampa in the department of Pasco (Table 2). Three potato
cultivars, for which the parameters needed for LB2004 were
previously obtained (4), ‘Tomasa Tito Condemayta’ (hereafter
referred to as ‘Tomasa’), ‘Yungay’, and ‘Amarilis-INIA’ (hereafter
referred to as ‘Amarilis’), were planted in four experiments for a
total of 12 epidemics. Cv. Tomasa has been reported as highly
susceptible to P. infestans, cv. Yungay as moderately resistant, and
cv. Amarilis as resistant (13). Data from the field experiments had
not been used previously for estimating model parameters.
DISCUSSION
The LB2004 version of LATEBLIGHT accurately predicted the
effect of weather and specific levels of host resistance on late
blight epidemics occurring in the Andes of Peru. The approach of
measuring LGR, SR, and LP of potato cultivars infected with
isolates of a certain clonal lineage of P. infestans under controlled
conditions (4) and then using the experimental results as parameters
of LB2004 proved to be effective. The validated model can
be used for educational and research purposes to illustrate concepts
and to generate hypotheses about disease management using
host resistance and manipulation of environment and initial inoculum.
The domain of applicability of LB2004 is defined by (i)
potato cultivars with similar resistance levels to P. infestans to
those of cvs.