Agriculture Reference
In-Depth Information
area duration, HAD) and yield compared with disease severity assessments.
Waggoner and Berger (1987) suggested that a logical adaptation of HAD (or
AULGAIPC) would be to integrate radiation by green tissue to give healthy area
absorption (HAA). Here, yield loss was related not just to disease intensity but also
to crop physiological variables. Traditional single-point, multiple point and integral
models such as AUDPC based on disease intensity do not give a complete
description of the disease-yield loss relationship, as crop yield is determined by
the magnitude of photosynthesis, a function of HAD (or AULGAIPC) and HAA.
Finally, it should be remembered that a complete disease-yield loss relationship
should also take account of economic thresholds for crop loss due to disease and the
assessment of any loss in crop quality.
2.6.2 Reference points, terms and concepts
In describing crop-yield loss relationships, it is important to establish reference
points, terms and concepts in order to standardize communication between workers.
Zadoks and Schein (1979), Campbell and Madden (1990c) and Nutter
et al.
(1993)
reviewed concepts and terminology for crop losses and differentiated between
potential losses (in the absence of control measures) and actual losses in crops, the
latter being sub-divided into direct (loss in quantity or quality of yield) and indirect
(the economic or social impact of losses). Similarly, yield was divided into
attainable yield (when crops were grown under optimum conditions), primitive yield
(when no disease control was applied), economic yield (highest net return on
expenditure), actual yield (obtained using disease management programmes) and
theoretical yield (obtained using calculations based on crop physiology or crop
growth simulation models). The difference between actual and attainable yield was
the method used by the Food and Agriculture Organization (FAO) to report crop
losses; most disease management programmes aim to close the gap between these
two yield concepts (Fig. 2.10).
2.6.3 Statistical and experimental methods
The assessment of yield loss is carried out using statistical and experimental methods.
Statistical methods were reviewed by Chaube and Singh (1991) and Parry (1990);
such methods utilize reports of disease incidence, estimated crop losses and yield for
analyzing yield losses. The accuracy of such methods is obviously inferior to that
obtainable from scientifically designed experiments. Briefly, statistical methods
involve the following: analysis of yields in relation to estimated disease incidence
over many seasons (but other factors such as weather, pests, farming practice and
plant varieties must be taken into account); comparisons of expected and actual yields
when it is known that a pathogen is the major cause of yield loss; yield analysis
before and after control measures are applied; and the use of holistic synoptic
methodology which involves grower questionnaires or national disease surveys by
agricultural officers to gather information on particular diseases (e.g. severity, date of
appearance, weather conditions, varietal susceptibility, estimated crop loss).
