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Description
of economical important pests in onion cultivation
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Onion
fly
Depending on the weather, the first oviposition takes place from
April to May (2. generation July/ August). The maggots eat in the
neck of the onion, which usually causes bacterial soft rot after
a short time (sulphurous smell). The common, preventive application
of granulate should be realized in any case, since the omission
of a general control and insecticide coating of the seeds may quickly
lead to new mass spread.
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Botrytis
leaf blight
Especially when the weather is cool and moist, longish, white specks
of different size appear, beginning from the weather side. These specks
are at first 1 - 2 mm in size and of a white-green colour merging
indistinctly to the colour of the healthy leafage. Later they are
clearly delimitated and slightly caved. They damage the onion by destroying
the assimilation surface. This can easily be confused with damage
by hail (see neighbouring populations). |
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Nematodes
Too close crop rotation leads to the spreading of stem nematodes
(hosts: pea, carrot, spinach, stellaria media, polygonum ariculare).
Non-hosts: wheat, barley, maize, clover, Spanish trefoil, lettuce.
Early symptoms: focal appearance of compressed plants with twisted,
swollen foliage. Later the onion base is often ruptured. A control
is hardly possible, only disinfection of the soil. Important: observance
of crop rotation
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White
rot
A white mycelium with black sclerotia (0,5 - 1 mm) develops around
the root. The fungus is very aggressive, and once the infestation
has begun, it very quickly destroys the whole plant. This disease
is not transmitted by the wind, but spreads with soil, where it
can persist for eight years.
Extensive infestation makes onion cultivation impossible
Important: organized crop rotation and field hygiene
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Thrips
When they exist in large numbers (warm and dry weather), these insects
cause damage to the assimilation surface. As, at cool weather, the
thrips retire to the leaf sheath, they can be recognized there at
first. For the prevention of mass spread they have to be controled
in time with the appropiate measures. |
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Neck
rot
The infection takes place in the stage when the second - forth leaf
develops. However, the symptomes i. e. the damage is first noticed
in the storehouse. The outer skins of the neck tissue may become
glassy and soft. Later a gray powdery-appearing mold may appear
under the dry skins. Soft rot may follow. A combined coating of
the seeds prevents this disease.
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Downy
mildew
When the conditions for infestation are favourable, it comes to an
explosive spread in the onion population. This disease commonly appears
in spots in the field, mostly in wind-protected places. Long oval,
pale green discolourations show the infestation. Often a grey-brownish
mold can be observed. Especially when the weather is hot and sultry
and the sky is clouded, the plants should be observed. Sunny and dry
weather impedes the development of the disease. At present a preventive
treatment with contact fungicides (e. g. Maneb) is recommendable.
Systematically acting fungicides are on the market. |
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Pink
root rot
Recently, under warm weather conditions, the pink root rot has appeared
more frequently crop rotation!). In the early stages of infestation,
the central vascular bundles have a reddish colour, which can be
observed in torn roots. Later the whole root becomes pink and dies.
Although there may be an intensive development of new roots, the
plants are weakened. The control of this fusarium-related fungus
is very difficult. To ensure a good yield, the control should be
realized in time. Observe your plants carefully and ask for expert
advice in the case of doubt.
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Onions
