Agriculture Reference
In-Depth Information
rain-shields for wind-dispersed spores or beakers for splash-dispersed spores can
be used. Concentration gradients can be measured with volumetric samplers such as
rotorods; vertical sticky cylinders can be used effectively for large spores, such as
those of
B. graminis
. Rain-activated switches can be used to confine sampling to
periods of rainfall for spores released by rain (Fitt
et al.,
1989) and sampling can be
confined to specific times for spores with known diurnal or seasonal periodicities
(Fitt and McCartney, 1986). Conventional spore samplers usually use microscopy to
quantify concentration or deposition. This can be time-consuming and often restricts
the number of samples that can be collected. Spore sampling methods based on the
use of serological or molecular pathogen diagnostics are being developed
(McCartney
et al.,
2003; Ward
et al.,
2004). A spore sampler that collects spores
directly into microtitre wells, developed by the Burkard Manufacturing Company
(Rickmansworth, U.K.) (Wakeham
et al.,
2004), has been used to quantify
Mycosphaerella brassicicola
(cause of ringspot of cabbage) inoculum using
monoclonal antibodies (Kennedy
et al.,
2000). PCR-based diagnostics have also
been used to detect air-borne inoculum of oilseed rape pathogens (Calderon
et al.,
2002; Freeman
et al.,
2002).
The disease component of disease gradients has been measured as numbers of
lesions, numbers of infected leaves, numbers of infected plants, the percentage leaf
area affected or the percentage of the population of plants which is affected (Fitt
et al.
,
1987). For most diseases, only some of these measurements are appropriate; for
example sorghum downy mildew (causal agent
Peronosclerospora sorghi
) infects
plants systemically so cannot be assessed on individual leaves and barley leaf blotch
lesions merge so that they cannot be assessed individually (Bock
et al.,
1997). When
there is a choice of methods available, the selection of the method of assessment may
be influenced by the objectives of the work. Furthermore, it should be appreciated that
whether disease is measured as incidence or severity affects the form of the gradient
since incidence and severity gradients measured simultaneously can have different
slopes (Minogue, 1986). The advent of molecular diagnostic methods (McCartney
et al.
, 2003; Ward
et al.
, 2004) could change our view of crop disease assessment. For
example, such methods have the potential to quantify the amount of pathogen DNA,
and perhaps the biomass, in leaf tissue (see also Chapter 2).
The choice of distances (
x
) at which to assess spore numbers or disease is
influenced by the geometry of the source, the scale of the gradient and the objectives
of the investigation. Sources can be classified as point, line or area sources
(Gregory, 1968, 1973). Point sources may be individuals or small groups of infected
plants; ideally a point source should have a diameter of less than 1% of the length of
the gradient although the diameter is often 5-10% of the length in practice (Zadoks
and Schein, 1979). Line sources may be hedges containing infected alternative host
plants or strips of a susceptible cultivar. Area sources may be infected fields,
although such area sources may become point sources if the distance over which the
gradient is measured is kilometres rather than metres. Gradients from sources above
ground level are generally less steep than those from ground level sources.
For measuring gradients a minimum of five distances should be sampled. Ideally
there should be at least 10 distances, selected on a logarithmic scale with more
samples near to the source for gradients within crops, and more distances for gradients
