Agriculture Reference
In-Depth Information
CHAPTER 2
DISEASE ASSESSMENT AND YIELD LOSS
B.M. COOKE
2.1 INTRODUCTION
Disease assessment, or phytopathometry, (Large, 1966), involves the measurement
and quantification of plant disease and is therefore of fundamental importance in the
study and analysis of plant disease epidemics. Nutter
et al.
(2006) distinguished
between disease assessment and phytopathometry, the former being defined as the
process of quantitatively measuring disease intensity and the latter as the theory and
practice of quantitative disease assessment. The importance of accurate disease
assessment methods was identified in early reviews on phytopathometry and crop
loss assessment by Chester (1950) and Large (1966). Kranz (1988) stated that
without quantification of disease no studies in epidemiology, no assessment of crop
losses and no plant disease surveys and their applications would be possible. Lucas
(1998) advanced the idea that disease assessment includes a number of interrelated
activities, such as the future progress of the disease, disease diagnosis, forecasting
and crop loss. Strange (2003) maintained that that the measurement of plant disease
and its effects on crop yield, quality and value are crucial for control priorities.
Traditional methods of disease assessment, such as the use of pictorial keys derived
from standard area diagrams to evaluate disease severity on a 0-100% scale, have
now been joined by several new approaches made possible by rapid advances in
computer technology. In addition, modern assays using immunological and
molecular techniques for the identification, detection and quantification of plant
pathogenic organisms are used. Other new approaches to phytopathometry have
evolved in which remote sensing, image analysis and the detection of crop stress
caused by disease (using changes in chlorophyll fluorescence and foliage temperature)
are involved.
The relationship between phytopathometric data and yield loss has always
suffered from a number of confounding factors and, as a result, many authors
including James (1983), have criticized the lack of reliable estimates of crop losses
due to plant disease. Chiarappa (1981) defined a crop loss model as 'a mathematical
method used to describe the relationship between yield reduction and the intensity of
harmful organisms'. It is therefore not surprising that current papers on crop loss
assessment deal with recent developments in modelling crop losses. Hughes (1996)
dealt with incorporating spacial patterns of harmful organisms into crop loss models;
Madden and Nutter (1995) discussed this and other developments that have changed
our understanding of the disease-yield loss relationship, such as healthy leaf area
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