Agriculture Reference
In-Depth Information
surface (deposition methods). The correlation between the two methods is poorly
understood and will obviously depend on meteorological factors. Concentration
methods involve sophisticated spore traps with a power source (e.g. the original
Hirst Volumetric Spore Trap), whereas deposition methods often comprise simple
sticky, horizontal or vertical surfaces exposed to the air under a rain shelter
(Wheeler, 1976). Rain-dispersed spores can be effectively caught in funnel traps
positioned within the infected crop (Deadman and Cooke, 1989); these are then
emptied after rainfall and the spores counted on a haemacytometer slide. Methods
used to trap spores in this way therefore involve estimates of spores using
microscopy, or colony counts in culture or on living plants used themselves as spore
traps. An extension of the latter is the use of trap nurseries and mobile nurseries
(Eyal
et al.,
1973), in which sets of genotypes are assembled that carry specific
resistances to the target pathogen in different geographic locations. Standardized
methods of sowing and disease assessment are used and samples sent to a testing
centre for virulence identification, usually as part of a race survey. Other indirect
methods of assessing disease include measuring the effect of the pathogen on host
parameters such as (for cereals) stunting, increased or decreased tillering, root
growth, premature or delayed ripening and reductions in ear number, grain number,
size and quality. Deadman and Cooke (1987) used such methods to assess the effects
of net blotch disease on the growth and yield of spring barley.
It is often the case that data from the visual assessment of plant disease severity
do not correlate with the amount of fungal biomass colonizing host tissue; this lack
of correlation inevitably leads to inaccurate disease-yield loss relationships (section
2.6.1). Whereas diseases such as powdery mildew, which has a superficial
ectotrophic growth habit on the host, may well show a close correlation between
visual assessment and tissue colonization, most other diseases, where the pathogen
is more invasive of the host tissue, are unlikely to show such a relationship. In order
to test these assumptions, several workers developed more precise techniques of
quantifying fungal biomass within host tissues, either by measuring fungal chitin
(Ride and Drysdale, 1972; Parker and Royle, 1993) or ergosterol (Griffiths
et al.,
1985; Gunnarsson
et al
., 1996). Chitin is not found in plant tissue but is a principal
component of fungal cell walls and, similarly, ergosterol is a fungal membrane-
specific component. Thus, the chemical assays used for these biomarkers provide
sophisticated quantitative techniques for the indirect assessment of disease severity
in plant tissue. Strange (2003) provides a good overview of the methodology used in
these techniques. Other indirect methods use spore production as a measure of
severity; Gough (1978) described a method for evaluating wheat cultivar response to
Septoria tritici
based on pycnidiospore production from soaked leaf segments using
haemacytometer counts.
Indirect methods of assessing disease incidence traditionally rely on the use of
selective agar media on which infected plant materials are plated out. Methods for
quantifying seed infection in this way are described by de Tempe (1964) and
Neergaard (1979), whilst Pettitt
et al.
(1993) described a technique using potato
dextrose agar supplemented by benomyl (Benlate fungicide, Du Pont U.K. Ltd.) for
the improved estimation of the incidence of
Microdochium nivale
in winter wheat
stems; the technique exploits the widespread incidence of benomyl resistance in
