Agriculture Reference
In-Depth Information
It is essential that individual assessments are made for all disease risks.
Although, for example, about 20% of winter wheat crops in England would benefit
from treatment against eyespot (Bill Clark, ADAS, UK, personal communication)
there are seasonal and rotational variations. In some areas routine treatments for
eyespot control were economic only in three of the 11 years between 1985 and 1995
(M. Nuttall, Morley Research Centre, UK, unpublished). This run of data was
collected from experiments within commercial second wheat crops sown during late
September or early October (i.e. those supposedly at high risk of eyespot infection),
in three or four-year rotations including combinable crops and sugar beet. In these
situations, sowing date of the first wheat crop is often delayed by late harvest of the
root crop. This reduces the risk of severe eyespot developing in the first crop and,
thus, restricts the amount of inoculum available to infect the second crop.
Application of epidemiological principles also improves assessment of risk at the
time when a fungicide spray decision has to be taken. Wet conditions over the
previous six weeks will increase the likelihood that a high level of latent eyespot
infection is present in the crop and will thus make it more likely that a spray will be
required. In practice, information of this type has been used in western Europe to
modify simple threshold-based decisions and determine spray need and timing for
eyespot control.
For powdery mildew and yellow rust, there are compelling reasons why fungi-
cides should be applied to winter cereals to reduce inoculum at the start of stem
extension during the spring. When conditions are favourable, these diseases often
have a latent period of less than 10 days. This is substantially less than the phyl-
lochron (time between emergence of successive leaves) so that impetus of the
epidemic builds up on each successive leaf layer.
Septoria tritici
on the other hand,
not only has a long incubation period (typically at least three weeks - well over the
phyllochron) but also relies on rain or wet conditions before spore dissemination can
occur. The length of the pathochron (Beresford and Royle, 1988) reduces the
potential for the disease to build up on upper leaves until stem extension is complete
and final leaves are available for infection.
This knowledge of disease epidemiology indicates that there is little point in
applying fungicides for septoria control during early stem extension unless a period of
wet weather occurs and suggests that disease control on the lowest (3
rd
and 4
th
) leaves
may only have benefit in open-canopied crops (Cook, 1997). Leaf 3 does not usually
emerge to become infected until late April at the earliest in the UK and it is thus the
development of disease from this time onwards which is of most importance. Data
from disease control experiments and annual surveys of wheat crops in the UK
indicate that sprays applied in May to protect upper leaves from late stem extension
onwards, rather than earlier, provide the best control of septoria tritici blotch (e.g.
Thomas
et al.,
1989; Cook
et al.,
1991). Under European conditions, there is little
benefit to be obtained by reducing inoculum of the pathogen during the winter as it can
never be completely eliminated and, if conditions are favourable, even a low level of
inoculum can result in a very high rate of disease development. Farmers often attempt
to reduce the inoculum of
S. tritici
by applying fungicides earlier than necessary.
Epidemiological knowledge confirms that weather conditions during and immediately
after stem extension are critical for the epidemic. Additionally, ascospore infection can
