Agriculture Reference
In-Depth Information
duration, radiation interception and time of infection, and Gaunt (1995) reviewed
technologies in disease measurement and yield loss appraisal.
Of paramount importance in disease assessment and yield loss appraisal is the
standardization of concepts and terms in order to improve communication between
plant pathologists and across scientific disciplines (Nutter
et al.,
1991). Standard-
ization would also permit industry to compare assessment data from several field
trials for product evaluation (Watson and Morton, 1990). Consequently, the reader is
referred here to glossaries of disease assessment terms and concepts (Nutter
et al.,
1991) and terms and concepts for yield, crop loss and disease thresholds (Nutter
et al.,
1993).
2.2 WHY ASSESS DISEASE AND YIELD LOSS IN PLANTS?
The assessment of the amount of disease on a plant or a crop of plants is essential in
any quantitative epidemiological study. Jones and Clifford (1978) and James (1983)
identified a number of important reasons for phytopathometric and crop loss
measurements, the most important of which must surely be that if we are not in a
position to estimate the losses from diseases, then how can we decide rationally on
how much to spend on control? Other reasons identified include the importance of
disease survey data to farmers, plant breeders, fungicide manufacturers, economists
and government agencies in determining the priorities for allocating resources and
timing control measures. Finally, researchers and extension workers require precise
methods for evaluating their experiments, particularly plant breeders where potential
resistant germplasm is being screened. Unfortunately, as pointed out by Campbell
and Madden (1990c), there is often a lack of a perceived need for estimates of yield
loss by some scientists and administrators; this translates into a lack of funding for
the research needed to provide such estimates, particularly at regional and national
levels. On a global basis, James (1983) pointed out the urgent need for effective crop
loss assessment programmes in the context of a world food situation that continues
to be precarious, especially in those developing countries which can least afford to
lose crop yield. Oerke and Dehne (1997) calculated actual and potential crop losses
of eight major food and cash crops by evaluating data from the literature and field
experiments from 1965 onwards. Yield limiting factors identified were water (the
most important), genetic yield potential and adaptation, and crop losses due to plant
pathogens, pests and weeds. Disease assessment and crop loss appraisal will become
especially important in sustainable systems of crop protection, where critical
evaluation of disease levels is required in order to assess the effectiveness of
proposed low-input, environmentally friendly strategies, such as the use of cereal
cultivar mixtures.
2.3 METHODS USED IN SAMPLING PLANTS FOR DISEASE
Any sampling method used in disease assessment must be random, representative
and objective and, depending on the disease involved, can be destructive or
non-destructive (Jones and Clifford, 1978). Until recently, little research in plant
