Agriculture Reference
In-Depth Information
onto the upper leaves is a requirement for severe disease. Short and thin crops are
therefore often more severely affected than taller crops with dense canopies. Disease
spread can also occur by dew or by wind movement of leaves (Lovell and Parker,
1997). These mechanisms allow disease to spread from one leaf layer to another
when layers overlap in short-internode cultivars.
Manipulation of the soil environment can also affect disease development.
Liming and irrigation, for example, greatly affect the physico-chemical and
microbiological environment of the soil and thus influence the risk of disease from
soilborne pathogens. Common scab (caused by Streptomyces scabies ) of potatoes,
for example, is favoured by a high pH, and club root (caused by Plasmodiophora
brassicae ) of brassicas by low pH. In a rotation containing both potatoes and
brassicas, it would be advisable to lime before the latter crop and not before the
former.
Fungi that produce motile zoospores are likely to be favoured by irrigation. This
leads to a problem for potato growers wishing to control tuber scab. Common scab
prefers dry soil conditions and can be controlled by irrigation during the period four
to six weeks from the time of tuber initiation. This, however, favours the
development of powdery scab (causal agent Spongospora subterranea ). Farmers
trying to control the former by irrigation can often reduce the benefits of their
strategy by aggravating the latter.
Sclerotium rolfsii, the cause of stem rot of peanuts, is favoured less by
continuous wetness than by the alternation of wet and dry conditions that irrigation
provides. In such a situation, irrigation can greatly impair the disease control
achieved by fungicides (Davis, et al., 1996). However, not to irrigate would reduce
yields and so jeopardize the profitability of the crop. The grower seeking to operate
an ICM system thus faces the dilemma of balancing the irrigation needs of the crop
with the need to reduce disease by limiting soil moisture.
Where it is not possible to modify the environment, reducing crop exposure to
risk is a valid alternative. Delay of sowing date can reduce the risk of over-wintering
yellow rust in winter wheat - temperatures usually being too low for infection to
occur by the time the crop emerges. With mildew on spring barley, on the other
hand, delayed sowing increases the risks of a damaging attack on the young plants
which emerge when temperatures are rising in late spring.
Where a mixture of pathogens is present, attempts to adjust the timing of
husbandry operations to reduce disease incidence may be made more difficult by the
different epidemiological requirements of the pathogens concerned. In potatoes, for
example, watery wound rot (caused by Pythium ultimum ) is favoured by high
temperatures, and gangrene (caused by Phoma exigua f. foveata ) by low ones.
Lifting of the crop too early (when the weather is too warm) can increase risks of
losses due to the former disease; lifting too late (when it is too cold) can aggravate
the latter.
Husbandry practices employed for purposes other than disease control can
sometimes increase the risks of disease and sometimes reduce it. The use of growth
regulators on cereal crops, for example, can increase disease incidence making it
easier for spores of Septoria spp. in wheat or Rhynchosporium secalis (the cause of
leaf blotch) in barley to be splashed on to the upper leaves (by shortening the crop)
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