Agriculture Reference
In-Depth Information
practices offer less effective methods of disease control. However, to prevent a
dangerous increase in inoculum levels, intensive cropping with a susceptible species
should be avoided.
(d) Sclerotinia
Sclerotinia sclerotiorum
differs from G.
graminis
in its ability to produce very long-
lived resting bodies, in its production of copious airborne spores and in its wide host
range amongst dicotyledonous crops. Once again, rotational practices are unlikely to
provide effective control of this fungus and other methods must be sought to reduce
inoculum levels. It has been suggested that, after a severe attack, a cereal crop
should be established by using minimal cultivation (graminaceous crops are immune
to infection). Sclerotia thus left on or near the soil surface will produce apothecia,
most of the spores from which will be deposited harmlessly on the cereal leaves or
on the soil, though some will be caught up by air currents to act as a source of
infection for other susceptible crops in the area. Eradication of inoculum from an
individual field will not protect a crop grown in that field from infection by such
airborne spores, but it will reduce the risk of a severe attack which is most likely to
occur close to the source where inoculum pressure is highest.
Adjacent susceptible crops and weeds can also increase the risk of infection.
Jerusalem artichoke
(Helianthus tuberosus),
for example, is very susceptible to
sclerotinia and can provide an inoculum source for other subsequent susceptible
crops grown in the same field or in neighbouring fields. In southern Lincolnshire,
UK, in 1991 (a year particularly conducive to the disease), severe infection occurred
in two rape crops adjacent to a field where a strip of Jerusalem artichokes had been
ploughed in five years previously, although other local crops were not affected
(J.M.Ll. Davies, ADAS Terrington, UK, personal communication). An example of
the importance of weeds was provided by Hims (1979). Severe sclerotinia in a crop
of oilseed rape was confined to a corner of a field adjacent to a wood where
hogweed (
Heracleum sphondylium
)
and cow parsley (
Anthriscus sylvestris
)
were
dying prematurely as a result of infection by the pathogen.
(e) Use of trap-crops
For some diseases, the effect of rotation can be enhanced by stimulation of resting
spore germination by growing a 'trap crop' host before the commercial crop. This
approach was used by White (1954) to reduce powdery scab (caused by
Spongospora subterranea
)
in potatoes by planting
Datura stramonium
to stimulate
germination of resting spores. However, such trap crops are unlikely to be used in
practice unless they provide a profitable crop in their own right.
( f) Soil sterilisation
While the eradication of soilborne inoculum of plant pathogens is very difficult in
agricultural systems, it may more effectively be achieved in commercial horticulture
