Agriculture Reference
In-Depth Information
Figure 14.2. Illustration of take-all progress curves for various levels of an experimental seed
treatment in a field experiment conducted in 1995, at Le Rheu, France. Symbols represent
observed means for each experimental treatment (control
and two rates of fungicide
∆
,
{
).
(From Schoeny and Lucas, 1999).
Similar approaches, using a similar type of model, have been applied to other soil-
borne diseases, including some with an aerial phase, such as eyespot (caused by
T. yallundae
) and sharp eyespot (caused by
Rhizoctonia cerealis
) on winter wheat
(Colbach and Saur, 1998; Colbach
et al.,
1997b). For both diseases, the impact of
the major components of cropping systems (crop rotation, soil tillage, wheat
management) was assessed by means of field trials, with several disease assessments
during wheat growth. Early sowing consistently increased disease incidence through
the primary infection cycle, whatever the disease. The frequencies of eyespot and
sharp eyespot were increased by high plant density and/or small numbers of shoots
per plant during the primary infection cycle. In contrast, in the secondary infection
cycle, the frequency of these diseases was decreased by small shoot number per
plant, that reduced late disease development at high density. For both diseases, high
doses of nitrogen increased disease levels through both infection cycles. However,
nitrogen fertiliser in the form of ammonium (
vs.
ammonium nitrate, i.e. 'mixed'
fertiliser) decreased eyespot levels as it does for take-all, but had the opposite effect
on sharp eyespot.
For eyespot, the model was also used to analyse the influence of crop residue
distribution on disease development and infection cycles (Colbach and Meynard,
1995). Differences in the amount and placement of crop residues were achieved by
varying crop rotation and soil tillage before the assessed winter wheat crop. When
the previous crop was a host crop preceded by a non-host crop, soil inversion
resulted in the burial of host residues, thereby decreasing primary infection risk.
However, if the previous crop was a non-host crop preceded by a host crop, soil
inversion carried the host residues back to soil surface, thereby increasing primary
infection risk. Secondary infection was not correlated with crop succession or soil
tillage.
