Agriculture Reference
In-Depth Information
unpredictability in the occurrence, direction and strength of the wind or rain.
Consequently, some experiments on dispersal have been done in controlled wind
tunnel or rain tower conditions (Fitt
et al.
, 1986); results obtained with these model
systems have then been compared with those obtained in field crops. In controlled
conditions, it is possible to replicate treatments in time but this is rarely feasible
under natural conditions. Decisions about replication, the size and shape of
experimental plots and of the inoculum source, the isolation of plots, the sampling
positions and the frequency and methods of assessment, are often difficult to
reconcile, especially if the objective is to compare treatments (e.g. cultivars or
fungicides) or to study development of gradients with time. However, knowledge
about the mode of spore dispersal (by wind or rain-splash) and thus the scale of the
gradient to be measured helps in making these decisions.
Experiments with wind-dispersed pathogens, such as
Puccinia polysora
(cause
of maize rust; Cammack, 1958),
P. infestans
(Minogue, 1986) or
B. graminis
(Welham
et al.
, 1995) have used point sources of inoculum to study horizontal
dispersal and disease gradients. When inoculum has been placed in the centre of
plots, disease assessments have been made at points on concentric circles around the
source. By contrast, Minogue (1986) placed inoculum at the end of long thin plots
and measured the potato blight gradients along the plots; this method does not
provide any information about two-dimensional disease spread and is only practical
if the prevailing wind direction is reasonably constant. An alternative method is to
use a line source of inoculum, perpendicular to the prevailing wind direction, and
measure the gradients along transects parallel with the wind direction (Vloutoglou
et al.
, 1995). If several transects through the same plot are used as replicates, they
are not true replicates because values are more highly correlated than they would be
in separate plots and experimental errors are underestimated. For splash-dispersed
pathogens, such as
S. tritici
or
R. secalis
, it is not necessary to use such large plots
and the problems with point sources of inoculum are fewer than for wind-dispersed
pathogens, because the scale of dispersal is smaller. The isolation distance between
plots or area of non-susceptible crop between plots required to prevent cross-
contamination is also smaller. However, it is still advisable to include uninoculated
control plots to assess levels of background contamination on the site.
The physical process of disease assessment may influence the dispersal of the
disease, particularly for diseases spread by readily detached wind-borne spores.
There can therefore be a conflict between the need to sample frequently enough to
obtain useful disease progress information and the need not to disturb the crop. The
use of long thin plots (Minogue, 1986), paths along transects (Vloutoglou
et al.
,
1995) or a ladder attached to a central pivot which can be rotated above plots
(Welham
et al.
, 1995) can minimise the effects of disturbance to the crop on disease
spread. For crops with relatively open canopies, such as maize, the risks of
disturbing plants during the assessment are less than for crops with closed canopies
(e.g. potatoes, barley) or with plants at high densities (e.g. linseed). For splash-
dispersed pathogens, assessments can be done when leaves are dry to minimise the
spread of pathogen spores.
Studies on vertical disease gradients for splash-dispersed pathogens have
sometimes involved assessment of disease on successive leaf layers up plants in the
