Agriculture Reference
In-Depth Information
by partial soil sterilization. However, the use of chemical sterilants will be unac-
ceptable in a sustainable system, and steam sterilization (once widely practised in
the glasshouse industry) is today too costly for most situations. At least in warmer
climes a possible alternative is offered by soil solarization - the use of the sun's heat
to achieve partial sterilization of soil covered by a polyethylene sheet. The process,
which has been successfully used in parts of southern Europe since the late 1970s
(Tjamos and Faridis, 1980), has been reviewed by De Vray (1995) and Katan (1994)
and has been the subject of international conferences, for example The Second
International Conference on Soil Solarization and Integrated Management of
Soilborne Pests, ICARDA, Aleppo, Syria, 1997. The technique not only reduces
pathogen populations by the direct effect of heat but also stimulates biological
processes which contribute to their control.
11.2.3 Crop residues as a source of inoculum
Infected stem and leaf debris act as a potent source of inoculum for many diseases.
Ploughing can reduce inoculum by burying debris but can bring its own problems.
Advice to leave rape stubble unploughed for sclerotinia control contradicts the once
standard recommendation that such stubble should be ploughed to reduce the
inoculum of
Leptosphaeria maculans
(teleomorph of
Phoma lingam).
Ascospores of
L. maculans,
produced in the autumn from cankered stems left on the soil surface
are more likely to introduce disease into nearby crops than are those of
S.
sclerotiorum
which are released in the spring when a covering cereal crop is likely
to be present to restrict their movements in air currents. Moreover, ploughing will
reduce inoculum of
L. maculans
more effectively than that of
S. sclerotiorum
as the
mycelium of the former fungus in the buried stem bases will not survive as long as
the buried sclerotia of the latter. A case could thus be made for ploughing rape
stubble where canker has been a problem but sclerotinia is absent, and for leaving it
on the surface after a severe attack of sclerotinia (in the former case the aim is
to reduce the amount of inoculum produced, in the latter it is to restrict the
dissemination of the inoculum.) Recently, however, there has been a move towards
leaving rape stubble unploughed until seeds shed during harvest have germinated;
this prevents ungerminated seeds being buried and providing a weed problem in
future years. By the time the volunteer plants have emerged and the field has been
ploughed, ascospores of
L. maculans
will already have been produced on the stubble
and could have infected young plants of the next season's crop in fields nearby. This
well illustrates the way in which an ICM practice employed to solve one problem
(weed control) can exacerbate another (disease).
The stem base pathogens,
Oculimacula yallundae
and
O. acuformis
(anamorphs
Pseudocercosporella herpotrichoides
f. spp.), the causes of eyespot of cereals,
provide an interesting example of the way in which the effects of inoculum level can
be over-ridden by other factors in the early development of an epidemic. Where
cereal follows cereal, splash-borne conidia, produced in abundance on stem bases
left after harvest, are likely to be of primary importance in infecting the subsequent
year's crop. It might be expected that the disease would be less severe after
