Agriculture Reference
In-Depth Information
crop (white leaf spot on oilseed rape; Inman, 1993) or sampled from the crop
(septoria tritici blotch on wheat; Shaw, 1987). Vertical spore dispersal gradients for
wind-borne spores have generally been studied with volumetric spore samplers, such
as rotorods, placed at different heights above the crop (McCartney, 1990a;
McCartney and Lacey, 1990; Vloutoglou
et al.
, 1995). The work on
Alternaria
linicola
(seedling blight; Vloutoglou
et al.
, 1995) in linseed is one of the few studies
which have combined successive measurements of both spore concentration
gradients and disease gradients in the same crop. It clearly demonstrates the effects
of background inoculum and the way in which secondary spore dispersal can flatten
primary spore dispersal and disease gradients with time.
6.3.3 Uses of spore dispersal and disease gradients
The uses of spore dispersal and disease gradients have been discussed by Fitt and
McCartney (1986), Minogue (1986) and Jeger (1999), so will only be summarised.
Measurement of disease or spore gradients can be extremely important for identifying
sources of disease, for identifying inoculum dispersal mechanisms, for assessing the
effectiveness of some disease control strategies and for interpreting the results of field
experiments.
Since the observation of a disease gradient implies the existence of a local source
of inoculum, gradient measurements can be used to identify inoculum sources. In the
Netherlands, disease gradient observations were combined with DNA fingerprinting
to identify sources of
P. infestans
in regional potato late blight epidemics
(Zwankhuizen
et al.
, 1998). The study identified infested refuse piles as primary
inoculum sources for establishing early infection foci in fields and infected organic
crops as secondary inoculum sources, resulting in the spread of at least two
genotypes within the region. From their observations the authors were able to make
recommendations on measures to reduce the risk of future epidemics. A vertical
gradient of cocoa black pod, with decreasing numbers of lesions with increasing
height above ground up to 3 m, suggested that the source of inoculum was at ground
level (McCartney and Fitt, 1998). Inoculum of
P. megakarya
was found to be
present in the soil and the gradient suggested that zoospores were being released into
ground water during rainfall, carried up onto the lower cocoa fruit in the splash zone
in large ballistic splash droplets and onto higher fruit in smaller air-borne splash
droplets.
Dispersal gradients can be used to infer inoculum dispersal mechanisms; shallow
gradients suggest wind dispersal and steep gradients imply splash dispersal. Primary
gradients of mummy berry disease (causal agent
Monilinia vaccinii-corymbosi
) in
blueberry crops were shallower downwind than upwind, suggesting that the
ascospores causing the infections were wind dispersed (Cox and Scherm, 2001). In
contrast, secondary disease gradients, caused by conidial infections, were generally
shallower upwind than downwind, suggesting that conidia might be dispersed by
insect pollinators. Gradients have also been used to assess the relative importance of
primary and secondary inoculum in disease development. Gradients of pod rot
(caused by
Botrytis cinerea
) at harvest of beans (
Phaseolus vulgaris
) were similar to
