Agriculture Reference
In-Depth Information
disease will be primarily determined by the level of inoculum in the seed, though
conditions at the time of crop emergence will have some influence on the number of
seedlings becoming infected.
Seedborne inoculum of
T. tritici
is effectively controlled by fungicidal seed
treatments. Reliance on routine application of such treatments can be questioned in
sustainable systems and is not allowed in organic production.
If, however, seed crops are
grown from seeds treated with a material which also affords protection against soilborne
inoculum it should be unnecessary to treat seed of the subsequent commercial crop,
unless it is to be sown in a field suspected to have been contaminated by spores from a
recently harvested, infected crop. This approach, although acceptable in ICM systems, is
no longer available to producers of organic wheat as current regulations preclude the
use of seed from crops that have themselves been treated with a synthetic fungicide
(unless suitable seed is unavailable).
Recent research has explored the use of more 'organically acceptable' seed
treatments such as acetic acid (Borgen and Nielsen, 2001) and mustard flour
(Borgen and Kristensen, 2001). At the time of writing, however, control of bunt in
organic wheat is dependent on the reduction in inoculum in the early years of
production of a seed stock and on the assumption that the inoculum has not
subsequently built up to potentially damaging levels. Oxley and Cockerell (1996)
found bunt in fields when the level of contamination in the seed sown exceeded 100
spores/seed. Testing of seed to be used in organic production is strongly
recommended to ensure that contaminated stocks are not sown. Because of the
difficulty of sourcing organically produced seed, organic farmers are sometimes
forced to use seed from crops which themselves were grown from conventionally
produced seed. The long-term availability of organically produced seed is a chal-
lenge for the organic cereal industry.
The approaches outlined for the control of bunt are also
applicable to the control
of diseases such as the cereal loose smuts (caused by
Ustilago
spp.) and leaf stripe
of barley (caused by
Pyrenophora graminea
)
.
Treatment of seed crops does not,
however, preclude the contamination of the seed by pathogens arising from other
sources of inoculum. The ubiquitous
Microdochium nivale
(which can cause serious
seedling blight) can infect cereal seeds during grain filling, whether or not seed
treatment was used on the parent crop. After a wet summer, it is advisable to have
wheat seed tested for the presence of this pathogen before deciding (even in an ICM
system) whether or not to use untreated seed.
The interest in reducing chemical inputs in arable agriculture has led to
increasing interest in the possible use of biological control agents (BCAs) to
suppress seed-borne inoculum of plant pathogens (see section 11.4.2 below).
11.2.2 Soil as a source of inoculum
Minimization of soil cultivations should be a primary objective of all sustainable
systems to conserve energy, soil and water. Minimal cultivation systems can,
however, be counter-productive in the context of disease control as they may leave
inoculum at or near the surface of the soil into which the crop is to be sown. Control
