Agriculture Reference
In-Depth Information
Inoculum sources should not, however, be ignored in an integrated control strategy.
When spring barley is sown close to a winter-sown crop, care should be taken to
ensure that winter and spring varieties have different genetic sources of host partial
resistance to mildew so that they are not susceptible to the same strains of the
pathogen (see also Chapter 10). A similar tactic should be adopted if it is necessary
to sow winter wheat adjacent to a set-aside field carrying self-sown plants of a wheat
cultivar susceptible to yellow rust (
Puccinia striiformis
).
The practical significance of the
r
and
Io
values in Van der Plank's equation is
well illustrated by reference to potato late blight (caused by
Phytophthora infestans
)
,
and discussed further by Mizubuti and Fry in Chapter 17
.
Careless husbandry, which
allows dense growth of infected haulm to develop on dumps of discarded tubers,
will provide so strong a source of initial inoculum (high
Io)
that the pathogen will be
able to take full advantage of the first period of favourable (warm and humid)
weather and an early attack of blight may be expected.
Complete elimination of inoculum will be impossible in any intensive potato
growing area and even isolated crops are likely to be infected by windblown spores
sooner or later. Careful hygiene on the part of the grower (and immediate neigh-
bours) may nevertheless so reduce the initial level of inoculum that the lag phase of
epidemic development is lengthened and two or three infection periods may be
necessary before the disease builds up to damaging levels. However, if there is a
long period of weather favourable to the development of the disease, the value of
r
in Van der Plank's equation will be so increased that a damaging epidemic will soon
develop, even if initial inoculum levels were low.
Because of the strain specificity of most obligate pathogens, volunteer plants of
the same species (and particularly those of the same cultivar) as the crop are most
likely to act as potential sources of inoculum to initiate an epidemic within that crop.
However, weeds can also play an important role as inoculum sources and effective
weed control has a part to play in the control of diseases in sustainable systems The
closer the botanical relationship between weed and crop, the greater the risk that
disease will spread from the one to the other. The spread of ergot from black-grass
(
Alopecurus myosuroides
) to wheat provides a good example of an obligate
pathogen spreading between species within the same family and much wider
taxonomic gaps can sometimes be crossed, especially by virus pathogens.
In a case encountered by one of the authors (DY) a very severe attack of
cucumber mosaic virus occurred in a newly built glasshouse on land that had never
before carried a cucumber crop. The grower, assuming that the 'virgin' land would
be disease-free, had omitted the usual soil-sterilisation practices. Unfortunately, the
soil happened to contain a large population of chickweed seeds, some of which
carried the virus. When the weed seeds germinated aphids very soon transmitted the
virus to the crop plants and a serious epidemic ensued. Given that steam sterilization
is now prohibitively expensive, any attempt to control such an epidemic in ICM or
organic production would encounter obvious difficulties. The use of chemical soil
sterilants would be frowned on in ICM, and prohibited in organic systems as the
latter would preclude the use of herbicides and insecticides. Mechanical control of
the weeds at a very early stage of growth would be the only feasible alternative.
