Major pests and diseases in onion

Transcription

Major pests and
diseases in onion
Leading in Allium,
inspired by you.
FOREWORD
Dear reader,
This brochure, produced by De Groot en Slot B.V. and Bejo Zaden B.V., provides
a description of the main pests and diseases in onion. A successful crop depends
partly on the prevention and/or effective control of pests and diseases. Onions can
be affected by many different diseases and pests both in the field and in storage,
many of which can adversely affect the yield and quality of a crop.
We work hard to develop new and even better quality hybrids, with resistances
to important pests and diseases. For advice about onion varieties and product
forms, please contact your local sales manager/crop consultant. Other useful
sources of information include the Bejo product catalogue and the website
www.bejo.com.
This publication does not include information on pesticides. Please ask your
pesticide supplier for approved treatments and their safety requirements.
We hope that this brochure wil prove useful, and wish you a succesful crop.
De Groot en Slot B.V. and Bejo Zaden B.V.
In this booklet we have presented the best information available to us. De Groot en Slot B.V.
and Bejo Zaden B.V. shall not be liable for any loss, damage or injury howsoever suffered
directly or indirectly in relation to the information in this booklet or the research on which it is
based.
© Copyright De Groot en Slot B.V. and Bejo Zaden B.V.
No part of this publication may be reproduced and/or published, redistributed or modified
usingprint, photocopy, microfilm or any other method, without the express prior written
permission of Bejo Zaden B.V. and De Groot en Slot B.V.
De Groot en Slot B.V.
Westelijke Randweg 1
1721 CH Broek op Langedijk
T +31 (0)226 33 12 00
E info@degrootenslot.nl
www.degrootenslot.nl
CONTENTS
Fungi
PURPLE BLOTCH (Alternaria porri)
BLACK MOLD (Aspergillus niger)
NECK ROT (Botrytis aclada(syn. Botrytis allii)
BOTRYTIS LEAF BLIGHT (Botrytis squamosa)
SMUDGE (Colletotrichum circinans)
FUSARIUM BASAL ROT (Fusarium oxysporum f.sp. cepae)
BLUE MOLD (Penicillium spp.)
DOWNY MILDEW (Peronospora destructor)
WHITE TIP (Phytophthora porri)
RUST (Puccinia allii)
PINK ROOT (Pyrenochaeta terrestris)
WHITE ROT (Sclerotium cepivorum)
STEMPHYLIUM (Stemphylium vesicarium)
ONION SMUT (Urocystis cepulae)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Bacteria
BACTERIAL SOFT ROT (Erwinia carotovora subsp. carotovora)
PANTOEA ANANATIS (Pantoea ananatis)
SOUR SKIN (Pseudomonas cepacia)
SLIPPERY SKIN (Pseudomonas gladioli pv. alliicola)
16
18
19
20
Viruses
ONION YELLOW DWARF VIRUS (OYDV – Potyvirus)
IRIS YELLOW SPOT VIRUS (IYSV – Tospovirus)
21
22
Insects
LEEK MOTH (Acrolepiopsis assectella)
WIREWORMS (Agriotes spp.)
WEEVIL (Ceutorhynchus suturalis)
ONION FLY (Delia antiqua)
ONION LEAF MINER (Liriomyza cepea)
THRIPS (Thrips tabaci)
23
24
25
26
27
28
Nematodes
STEM NEMATODE (Ditylenchus dipsaci)
ROOT LESION NEMATODE (Pratylenchus penetrans)
29
30
Deficiency disorders
MAGNESIUM DEFICIENCY
MANGANESE DEFICIENCY
31
32
Physiological damage
HAIL DAMAGE
SUN SCALD
TEMPERATURE STRESS
TRANSLUCENT AND LEATHERY SCALES
33
34
35
36
CHIMERA
37
Major pests and diseases in onion - 1
PURPLE BLOTCH
(FUNGUS)
Alternaria porri
This disease is found in all onion growing regions but is most virulent in hot,
humid conditions.
Symptoms
Small, watery lesions with a white centre can be seen on the leaves. As the lesions
grow larger, brownish purple rings containing spores are formed. The edges of
the flecks are reddish purple and these are ringed by a yellow zone. If the flecks
merge with each other they can affect the entire leaf, causing it to snap and die.
Older leaves are also the most vulnerable to this disease.
Development and infection
The fungus can remain in plant debris and waste heaps from where it can attack
the plant. The spores are formed during damp nights and when leaves are wet
for longer than 12 hours. When the leaf or leaf debris dries out, the spores are
spread to other leaves via the air. The first symptoms become visible between
1 and 4 days after infection. The optimum temperature for this disease is 25ºC
when the bulbs are most vulnerable.
Prevention and management
Apply extensive crop rotation. Ensure that the leaves don’t remain wet for long
periods by having good drainage and by planting less densely. Avoid excessive
irrigation. Fungicides can be used to combat this disease. The weather conditions
and the expected time that the leaves will be wet must be taken into account.
2 - Major pests and diseases in onion
BLACK MOLD
(FUNGUS)
Aspergillus niger
Similar to Penicillium, black mold is mainly a problem during storage, processing
and transport.
Symptoms
Below the dried skin a powdery, black mass of spores forms which is often
invisible on the outside. The spores can be present on each of the scales. Infected
scales first appear watery, after which white, fluffy mycelium appears on which
black spores quickly form. In advanced stages, the bulbs will start to shrivel.
Aspergillus is a common fungus mainly found in warm, dry regions. The fungus
occurs on many plant-based and animal products. It can survive on damaged
and dead plants and animals, fruit and other agricultural produce. Infection
usually takes place in the field and requires a leaf wetness period of at least 6
hours No symptoms can be seen on the plants in the field, except maybe a black
discolouration of the neck. The fungus penetrates the bulb via the neck and
attacks the scales. Wounds and damage on the outer scales and roots also allow
the fungus to enter the onion. Problems mainly occur at temperatures above
28ºC. Black mold is often followed by a secondary infection which causes the
bulbs to rot. The bulbs can also dry and shrivel.
Prevent damage and diseases on the leaf, which can allow the fungus to
penetrate the onion. Avoid damage to the bulbs during harvesting, storage and
transport. Keep the temperature (<15ºC) and humidity (<76% Relative Humidity)
low during storage and transport.
NECK ROT
(FUNGUS)
Botrytis aclada (syn. Botrytis allii)
Neck rot is a serious problem for onion cultivation throughout the world. The
disease arises during storage and transport and can cause considerable losses.
Symptoms
Bulbs generally have a soft neck. After cutting one open it can be seen that the
scales are turning brown, grey or black from the neck. Sometimes a white to grey
coloured fungal growth can be seen. The neck rot fungus can enter the bulb via
its base plate or damaged tissues and will display symptoms of the disease here.
In a more advanced stage the bulb will shrivel and become covered with a grey
fungal growth, sometimes this also has large sclerotia.
The fungus overwinters as sclerotia on rotting plant material or in the soil.
In the spring, the sclerotia produce asexual spores (conidia) that are easily
dispersed by the wind. These spores can infect the plant and remain there in a
latent state. No symptoms can be seen during the growing season. The disease
mainly spreads during wet periods. The critical infection period is between
ripening and harvesting. A crop that ripens quickly and is sufficiently dry during
the harvest gives few problems during storage because the fungus can’t enter the
bulb via its dry neck. The disease may manifest itself when the crop is in storage if
the crop was not sufficiently mature or was wet when harvested.
Remove waste heaps or cover them well. Avoid damaging or injuring the leaves
since these can serve as points of entry for neck rot. Be careful with nitrogen
during, but especially towards the end of cultivation. A crop that is still growing
rapidly will mature badly. Harvest at the correct time, when the weather is dry
and at least 50% of the foliage has died. Make sure not to damage the necks
during topping, since damaged necks are susceptible to neck rot infection. Sort
out and remove damaged or thick necked bulbs from the lot before storing. Dry
the onions as quickly as possible after harvesting. Forced drying is better than
drying in the open air. Ensure that there is good air ventilation in storage in order
to prevent condensation.
BOTRYTIS LEAF BLIGHT
(FUNGUS)
Botrytis squamosa
The different types of Botrytis are an important source of pathogens in onions
and this fungus occurs wherever onions are cultivated. Botrytis also causes
neck rot.
Symptoms
Botrytis leaf blight can be identified by its small, white spots with a light green
halo. In the initial stages the spots can’t be distinguished from those caused by
insects, hail or mechanical damage. Once infected with leaf rot, the leaf will break
at right angles to the lesion when pressed together. Over the course of time the
lesions become larger, merge together and can cause the leaf to die.
This disease mainly occurs towards the end of the growing season and is
especially prevalent on old leaves. Cool, humid conditions are most favourable
for this disease. Crops with dense foliage are particularly susceptible since they
remain damp longer. The fungus can survive the winter in the soil on plant debris
and as sclerotia. Waste heaps are also a source of infection.
It is necessary to apply an accurate spraying schedule for preventative purposes.
Removing or covering waste heaps reduces the chance of spreading spores.
Apply crop rotation.
SMUDGE
(FUNGUS)
Colletotrichum circinans
Symptoms
Colletotrichum is mainly a problem with white onions. Rings of black fruiting
bodies can be seen on the dried onions.
This disease mainly arises towards the end of the growing season and continues
to develop in storage. The fungus can remain in the soil for many years on plant
debris. During warm, humid weather the spores are released and can infect the
outer scale. After infection, new fruiting bodies form quite quickly, producing
more spores.
After harvesting the onions should be quickly dried until the outer scale is
thoroughly dry. Ensure good drainage of the soil and that the crop is free of
diseases. Apply crop rotation and consider cultivating yellow or red onions instead
of white in problematic regions
FUSARIUM BASAL ROT
(FUNGUS)
Fusarium oxysporum f.sp. cepae
Fusarium basal rot is a considerable problem particularly in subtropical regions,
however it can arise in milder regions during warm summers as well.
Symptoms
The first symptoms are the appearance of yellowing, twisted leaves. After this the
leaves die off starting at the tip. The entire plant can wilt during the early stages
of infection. Roots that have been infected turn dark brown and will rot. As the
infection develops a white fungal growth will become visible at the basal plate of
the bulb which in contrast with white rot does not contain sclerotia. If an infected
bulb is cut lengthwise, the bottom of the bulb and the lowest part of the scales
will appear to be watery and pale grey.
Fusarium is a soil borne fungus that can survive for a few years by means of
chlamydospores. Optimum ground temperature for the disease is around 25ºC.
Few symptoms are shown when the temperature is lower than 15ºC. The plant
can become infected at any stage. Damage to the roots or the bottom of the
bulb, caused for example by the onion fly increases the chance of infection. The
disease can be transferred to other plots by machines or onion sets and even
when in storage the Fusarium basal rot continues to develop and spread through
the lot.
Only varieties with a very high resistance to Fusarium basal rot should be
cultivated in infected fields. A crop rotation cycle of 4 years or more can
considerably reduce the chance of infection. Infected lots should not be stored
long term. However if this is necessary, then the temperature should be kept
below 4ºC.
BLUE MOLD
(FUNGUS)
Penicillium spp.
This disease is mainly found during storage and transportation.
Symptoms
Pale yellow, watery spots appear on the onions and these are quickly overgrown
with a bluish green fungal growth. If the onion is cut open it can be seen that the
scales appear watery and brownish grey. After a while the bulbs become soft and
wet rot can develop. Infected bulbs smell musty.
Penicillium is a common fungus that has a broad range of host plants. The
fungus can also be found on food items such as bread. The fungus grows best
under warm, humid conditions. Infection generally occurs through damage. In
damp conditions the fungus can also attack the bulb.
Avoid damage during harvesting. Ensure that the onions are thoroughly dried
after harvesting and prevent condensation.
DOWNY MILDEW
(FUNGUS)
Peronospora destructor
Downy mildew can be found in almost all the important onion growing regions. It
occurs mainly during periods of cool, humid weather and affects the quality and
quantity of harvests.
Symptoms
The initial symptoms to appear are pale green oval patches which in a later stage
become covered with violet grey spores. Foliage that has been affected becomes
more susceptible to other fungi such as Alternaria, which turns the foliage black.
The disease arises during humid weather conditions with the optimum
temperature for its development being between 15 and 20ºC. Early infection
can cause considerable damage. If conditions for infection remain favourable for
a prolonged period the disease can extend to infect large areas of the field. In
the end, plants that have been infected will die prematurely leading to reduced
harvests. Infected bulbs can lead to storage losses. The fungus can remain in the
soil or survive on plant debris, waste heaps and in onion storage facilities. As well
as this, overwintering onions and first year sets can often be sources of infection.
The fungus spores are spread by raindrops and wind and they will germinate and
infect the leaves under conditions of rain, dew and high humidity (RH>95%).
Once the fungus is present in the plant it grows systemically and can then
continue to produce spores.
Don’t plant infected first year sets. Ensure that crop debris is ploughed well into
the ground and cover waste heaps. Don’t irrigate excessively. Rotate crops every
3 or 4 years, if possible longer. Carry out preventative chemical treatments if
weather conditions favour the development of the disease.
WHITE TIP
(FUNGUS)
Phytophthora porri
Symptoms
This fungus causes small, light coloured spots to form on the leaf. The spots
rapidly increase in size when it is humid and cool (< 15ºC). The lesions are mainly
found on the upper half of the leaf, often surrounded by what is sometimes a
considerable green, watery zone. After a while, the affected areas and the leaf
tips die off. The parchment like appearance of the leaves and the white leaf tips
are typical characteristics of a Phytophthora infection. This fungus can cause rot
when the crop is in storage.
The oospores of this fungus are found in the ground and they come into contact
with the plant when water splashes up from the ground during heavy rain. Once
they are present on the leaves the spores need water for their development. Still
water can greatly enhance the spores’ development. The white spots become
visible approximately 14 hours after contamination. This disease is linked with
poor soil structure (compressed and poor drainage). Leek is also a host for this
fungus.
Avoid splash from irrigation on soil that is prone to Phytophthora infection.
Use extensive crop rotation in order to reduce the soils potential for causing
infections.
RUST
(FUNGUS)
Puccinia allii
Symptoms
Rust brown clusters of spores (pustules) form all over the leaf. The leaf tissue
surrounding the pustules turns a pale yellow. Later on in the season dark brown
spores (teliospores) can form. The damage caused to directly drilled onions is
mainly cosmetic, but when it occurs in garlic and bunching onions the loss in
quality and quantity can be high.
Rust can overwinter on leeks and garlic plants (plant debris) and infects new
propagating material in spring. The spores can be found on the pustules, from
where the spores are spread by the wind and are able to cause new infections.
A high relative humidity, moderate temperatures (10-21°C), high planting density,
high nitrogen and low potassium concentrations encourage this fungal infection.
Older crops are more susceptible.
Sanitation of equipment is important, as is the disposal of infected plants.
Healthy and strong foliage will reduce the risk of infection. Frost will inhibit the
development of rust, but does not kill the fungus.
PINK ROOT
(FUNGUS)
Pyrenochaeta terrestris
Pink root is an important problem for onions. This disease can cause considerable
reductions in crop yields in tropical and subtropical climates.
Symptoms
Plants that are heavily infected with pink root look as if they are suffering from
lack of water or as if they have a deficiency disorder. The roots of infected bulbs
first become light pink in colour and depending on the extent of the infection
the colour becomes more intense changing from pink to red to deep purple. After
this the roots shrivel up and die. Pink/purple discolouration can also sometimes
be seen on the skin of white onions.
The fungus remains in the soil in the form of dormant spores or in plant debris
of one of its many host plants. Infection occurs when onion roots come into
contact with the fungus. Because pink root doesn’t attack the bulbs base the
plant continues to develop new roots which are then also infected. When plants
are heavily infected there will be few or no healthy roots left, resulting in retarded
growth and reduced yields. Pink root doesn’t continue in storage.
Only cultivate varieties that have a high resistance in infected fields.
Apply a 4 to 6 year crop rotation using resistant crops.
WHITE ROT
(FUNGUS)
Sclerotium cepivorum
White rot is one of the most important and destructive fungal diseases of onions
and it causes damage wherever onion are cultivated worldwide.
Symptoms
Leaves become yellow and wither. When an infected plant is uprooted a thick
white fungal growth will be seen on the roots and the bottom of the bulb.
Innumerable small black sclerotia form in this fungal growth and on the affected
parts. Plants can fail completely because of this.
White rot fungus can stay in the soil on plant debris or as sclerotia. The sclerotia
can remain dormant in the soil for many years germinating as soon as onions or
a related crop are cultivated on the land again. White rot can spread quickly from
root to root. Cultivation machinery, planting material, crates or footwear can
also spread the disease. White rot doesn’t cause a lot of damage in the first year
of infection, but in the following years considerable numbers of plants may fail
because the number of sclerotia can increase rapidly.
Managing white rot is complicated, as the sclerotia can survive in the soil for up
to 20 years. Avoiding infection is essential. Follow good hygiene protocols on
the farm. Avoid soil from infected plots being carried by machines to other plots.
Correctly registering infected plots can prevent it from spreading to uninfected
plots. Inspect the propagating material (for example onion sets) for the presence
of white rot. If the disease is detected for the first time in a certain plot, remove
and destroy as many infected plants as possible to prevent the rapid spread of the
infection. Inundating or solarising (in subtropical regions) infected areas or entire
plots can reduce the number of sclerotia.
STEMPHYLIUM
(FUNGUS)
Stemphylium vesicarium
Stemphylium is a secondary pathogen that is found throughout the world but
causes the most damage in warmer regions.
Symptoms
The earliest symptoms are very similar to those seen with Purple blotch. An attack
begins with small, light yellow to brown, watery spots. After a while these
develop into elongated blotches that can merge with each other and attack the
entire leaf. The blotches are light brown in the middle, becoming darker towards
the edges. Black fruiting bodies can be seen clearly.
Infection mainly begins on old leaves. Dead or damaged tissues that may have
been caused by insects, fungus or simply dead leaf ends, provide points of entry
for the fungus. The disease can occur under varying circumstances but causes
considerable damage when the weather is warm and humid.
Good control of insects and other diseases is vital. This prevents weak tissue.
There are a variety of fungicides that can prevent this disease from causing
too much damage. In warm weather do not irrigate in crops with a slight
Stemphylium infection.
ONION SMUT
(FUNGUS)
Urocystis cepulae
Onion smut is a problem for onions that have been sown directly in moderate
climate zones.
Symptoms
Black spots or bands that are often a little swollen, can be seen on the cotyledons
and the first leaves. Later these tear open and a huge number of spores are
released. Infected plants suffer from retarded growth and in cases of extreme
infection will die off within a few weeks. The symptoms remain visible throughout
the entire season and will ultimately infect the bulb as well. Lesions on the bulbs
are often the point of entry for different secondary pathogens that cause rot.
The fungus spores can survive in the soil for up to 15 years and the very fine
spores are easily spread by wind, water, machines and infected plant material.
Generally speaking, only young leaves are susceptible to this disease. Mostly
infections occur during the first three weeks after germination, when the young
plants grow through the infected soil. Once a plant has been infected the
disease will continue on the newly formed leaves. The spores germinate when
temperature ranges between 10 and 22ºC, with activity decreasing rapidly under
higher temperatures.
Use healthy sets and transplants on smut-infested soil. This will minimise the risk
of infection.
BACTERIAL SOFT ROT
(BACTERIA)
Erwinia carotovora subsp. carotovora
Bacterial soft rot occurs in many types of vegetable crops. It can considerably
reduce the yield of onion crops both in the field and in storage.
Symptoms
Erwinia can infect the plants in the field. The leaves wilt and will dry out at a later
stage. If an infected plant is cut through, it can be seen that the middle of the
new bulb is completely slimy and stinks. The scales are soft and appear watery.
Later on a pale yellow to light brown slime will form. Affected bulbs are soft and
watery. When squeezed, the bulb oozes stinking fluid or slime.
The bacteria that cause problems when cultivating onions are only found in the
soil, surface water and on crop debris. Infection almost always begins via wounds
that have been caused by insect damage, hail, heavy rain and strong wind. There
can be many sources of infection such as through the fungal infections, leaf rot
and downy mildew, damage caused during growth or harvesting ie topping too
early or too short. Another source of entry can be when leaf axils begin to rot
following prolonged rain.
The bacteria mainly penetrate the bulb via wounds on the neck, but this is not
always the case. Water is necessary for infection. The warmer it is the faster the
infection progresses. The bacteria become inactive at temperatures lower than
3ºC.
BACTERIAL SOFT ROT
(BACTERIA)
Erwinia carotovora subsp. carotovora
Ensure that the soil has good structure, is well drained and is not compacted.
Control insects and any other pathogens. Avoid extreme foliage growth. Don’t
give too much nitrogen since a lush crop is fragile and especially vulnerable to
hail, rain and wind damage. Don’t irrigate too much after the bulb has developed.
Consider flattening the leaves mechanically if there has been damage from late
hail, to ensure that no water remains in the leaves. The crop must be completely
dry for harvesting and damage should be prevented as much as possible. The
onions should be stored in conditions that are cool and as dry as possible.
PANTOEA ANANATIS
(BACTERIA)
Pantoea ananatis
Besides a form of bulb rot this bacteria can also cause symptoms on the leaves
of onion plants. The bacteria mainly occurs in warm regions and can lead to
seriously reduced yields.
Symptoms
The plants can become infected in the field. The initial symptoms become visible
on the youngest, inner leaves. The leaves show watery lesions that quickly spread
over the leaf and white stripes and patches appear. The infection then spreads
towards the bulb where the infected scales become soft and watery. Infected
scales can vary in colour from pale yellow to brown. If the bulb is uprooted the
leaf easily loosens from the bulb and a thick fluid oozes out of the leaf and neck
of the bulb. A secondary infection with Erwinia often occurs.
The bacteria survive on crop debris as well as various weeds and can be
transmitted by thrips. However, little is known about which plants act as hosts.
Infection takes place in the same way as Erwinia.
Ensure good control of the thrips and onion fly populations. Keeping weeds down
will help reduce the bacteria populations too. In crop rotation try and restrict the
use of host plants for the bacteria to a minimum.
Photo: Department of Plant Pathology and Plant-Microbe Biology, Cornell University
SOUR SKIN
(BACTERIA)
Pseudomonas cepacia
Symptoms
Young foliage is particularly susceptible to this disease. The young leaves are often
attacked in their heart and turn light brown. Soft wet rot develops later. The
first characteristic after harvesting is a soft neck. The outer scale within the bulb
becomes watery and light yellow, later producing a yellowish brown slime. Bulbs
that are infected with Pseudomonas cepacia give off a typically sour smell.
The bacteria go from the leaves and the leaf axils to the neck of the bulb as it
is forming, where it remains latent. The first symptoms appear while the bulbs
are developing in the field. If the neck hasn’t been dried thoroughly during or
after harvest, then the disease moves via the neck into the bulb and infects the
outermost scale. The bacteria can then disperse itself very quickly throughout the
bulb especially with high temperatures.
See Erwinia (page 17). The same measures are applicable to all bacterial diseases.
SLIPPERY SKIN
(BACTERIA)
Pseudomonas gladioli pv. alliicola
Symptoms
In the beginning there are few external symptoms visible on the bulbs, the
only characteristic being a somewhat softer neck. The central scales (often one
or two) are soft and look as if they have been cooked. These scales later turn
brown. Affected bulbs will eventually dry out and shrivel. The bacteria can also
affect young plants in the field. Leaves wilt and will eventually die. The symptoms
displayed in the field are difficult to distinguish from those of Erwinia.
The infection happens in the same way as with Erwinia. The bacteria progress
from the infected leaf axils through the neck to the scales. After this the bacteria
seek a way via the infected scale to the bulb’s base. Via the base plate other
scales can be infected. Sometimes, if an infected bulb is squeezed the entire
core will fly out. This phenomenon is called Slippery Skin. Infected bulbs can
completely rot away within ten days at room temperature.
Prevention and cure
See Erwinia (page 17). The same measures are applicable to all bacterial diseases.
ONION YELLOW DWARF VIRUS
(VIRUS)
OYDV - Potyvirus
Symptoms
Onion Yellow Dwarf Virus occurs all over the world. The first symptoms of the
disease can be seen on the youngest leaves. The leaf is pale grey with yellow
streaks forming around the veins. Leaves are sometimes crinkled and flattened
and tend to fall over. Infected shallots or onion sets display more extreme
symptoms; the plants remain small, the foliage is curly, yellow, striped and bent.
The virus can be transferred by people and farm machinery. As well as this it
can be spread by insects such as the peach-potato aphid and other aphids. If an
aphid pierces an infected plant it can transfer the virus via its proboscis directly
to healthy plants (non-persistent transfer). The virus is not transmissible through
seed. Onion Yellow Dwarf Virus is related to the Leek Yellow Stripe Virus
(LYSV). However, it can only infect leeks with difficulty. Onions, shallots, garlic
and narcissi are also good host plants.
Keep the crops aphid-free. Remove infected plants from vegetative propagated
planting material of Allium varieties. Do not grow onions in the close vicinity of
other Allium crops.
IRIS YELLOW SPOT VIRUS
(VIRUS)
IYSV - Tospovirus
Symptoms
Iris Yellow Spot Virus is related to Tomato Spotted Wilt Virus (TSWV). The
initial symptoms include diamond shaped lesions on leaves and scapes. Lesions
often, but not always, have a green centre. Only seedlings can be killed by
the virus. Severely infected plants have a stunted appearance. Plants are more
susceptible to other stress factors.
The virus is transmitted by the onion thrips (Thrips tabaci). They carry the virus
for the rest of their lives. The virus can survive on various host plants such as
overwintering onions, iris, alstroemeria and leeks, but also in infected thrips.
Effective control of the thrips is essential. A healthy crop and good weed
management reduces the thrips population considerably.
LEEK MOTH
(INSECT)
Acrolepiopsis assectella
Symptoms
The leek moth’s larvae feed on the leaves creating transparent areas (windows).
Due to natural degradation of the membrane holes are then formed in the leaf.
A second generation of caterpillars can also eat into the bulb which can cause
neck rot. The first attack is usually first noticed in the rows on the perimeter of
the field.
The leek moth is an inconspicuous moth. Generally speaking there are three
generations each year and sometimes four. Usually the second generation gives
most damage to the crop. The moth eggs are dirty white in colour and are laid
over the leaf. After an average of eight days the eggs hatch and the greyish white
to green larvae begin to bore into the leaf. The caterpillars are present from the
middle of May until the middle of September, with a short interruption in June.
The leek moth prefers warm, dry summers. Besides onions, other crops from the
Allium family act as host plant for this insect.
No preventive measures are known. Carry out a chemical treatment when the
leaves are damaged.
WIREWORMS
(INSECT)
Agriotes spp.
Symptoms
Death of seedlings and young onion plants.
The adult wireworm is the click beetle (Elateridae family) which lays eggs on
rough grassland areas. These click beetles are frequently found on land used
to grow grass or grass seed. If this grassland is ploughed and subsequently the
plot is used to grow onions the wireworms present in the soil can cause a lot of
damage to the roots of the young plants. It can take as long as 3-5 years before
the wireworms pupate into an adult click beetle.
Control the population of adult click beetles to reduce the wireworm population
(larvae). There is a method that uses lures to attract the click beetles. Monitoring
of the lures enables optimal control of the population. To detect the presence of
wireworms, the soil can be sampled and tested.
WEEVIL
(INSECT)
Ceutorhynchus suturalis
Symptoms
The weevil tunnels holes in the onion leaf and lays its eggs in these holes. When
the larvae pupate into adults they turn orange. The damage they cause resembles
that of the leek moth, but the weevil bores more holes and eats in patches. The
weevil also only has one generation per year.
There are no known preventive measures. As soon as insect damage is noticed,
apply a crop treatment.
ONION FLY
(INSECT)
Delia antiqua
Symptoms
Loss of plants.
The first generation of the onion fly lasts from the beginning of May until
mid June. The adult fly lays her eggs on the young plants close to the ground.
The larvae that emerge from the eggs eat their way through the young plant
causing it to die. The larvae then move on to the neighbouring plant. The second
generation, from July until September, can damage the bulbs and also creates
points of entry for other pathogens. The larvae pupate after a number of weeks.
The pupae are reddish brown in colour and the pupae from the first generation
become the fly of the second generation. The fly has 2 to 5 generations a year,
depending on the climatic zone.
To combat the first generation use seed that has been coated with insecticide.
By using a surplus of sterile males in the plot, the majority of the eggs will not be
fertilised or produce any larvae.
ONION LEAF MINER
(INSECT)
Liriomyza cepea
Symptoms
The small pale grey larvae of the insect burrow in the leaves. The miners tunnel
irregularly and are whitish or light green in colour. As the larvae grow the tunnels
become wider. The damage caused by a single larva is relatively insignificant
whereas large numbers can considerably weaken or even destroy young plants.
Infected leaves are more susceptible to damage from the wind and other
pathogens.
The adult female punctures the leaf and lays her eggs inside. The larvae hatch out
after a few days and go through a number of different stages before becoming
a fully grown leaf miner. It is mainly in the last stage that the leaf miner can
be found under the surface of the leaf. Ultimately, the insect leaves the leaf to
pupate. Two generations can develop in one season. The neck and head of the
onion can be damaged while the second generation is growing and developing.
Leaf miners have a wide range of host plants, including many weeds. Removing
plant debris and applying weed control considerably reduces the chance of
infection.
THRIPS
(INSECT)
Thrips tabaci
Symptoms
Infection starts with pale green dots on the leaf that turn into silver grey blotches.
The thrips itself (larvae) are also sometimes visible. Small, pale brown elongated
insects that are mainly found in the leaf axils on the youngest leaf tissue of the
inner leaves. Thrips can also cause cosmetic damage on neck and scales of the
bulb.
Thrips overwinter on bulbs, as pupae (on crop debris or in the soil) or on other
host plants. The population can increase explosively, especially during hot, dry
weather. An average temperature increase from 15ºC to 20ºC doubles the
population. The population consists of: 1-3% adult thrips, 15-30% larvae,
60-75% eggs. Sucking damage in the leaf tissue results in white to grey spots and
death of the damaged tissue. Damaged leaves are more susceptible to secondary
pathogens. In addition, thrips can also transmit Iris Yellow Spot Virus.
Controlling thrips is difficult above a temperature of 25ºC. It is important to start
pest control at an early stage (from a soil temperature of approx.11.5ºC). Control
early in the season is recommended after a mild winter or early spring. Regularly
inspect the shafts of a number of plants and/or use blue sticky traps between the
plants to indicate the size of the population. Before starting the control measures
consider factors such as neighbouring plots of host plants (onions) and/or road
sides that have been mown. Apply treatment at a temperature below 25ºC
and with low intensity sunlight, as the insect avoids bright sunlight. A healthy,
vigorously growing crop is less susceptible to thrips damage. As thrips also live
on many types of weeds, effective weed control can considerably reduce the
infection pressure. Regular rainfall also contributes to a temporary reduction
of the thrips population. Thorough ploughing can also reduce the problem in
subsequent years. Adjuvants, insect adhesives and attractants can help to control
thrips that are difficult to reach.
STEM NEMATODE
(NEMATODE)
Ditylenchus dipsaci
Symptoms
The leaves of the young plants are twisted and seriously deformed. Later, the
infected plants will be stunted and blue in colour. The leaves can be thickened
and fragile in structure. Infected bulbs are greyish and soft to the touch. The inner
scales swell up more and have a white, softened structure. The roots rot away
and the plants start to rot. Infected plants and bulbs give off a distinctive smell.
At the start of the season larvae invade the young tissue of the germinating
plants below the soil. The nematode can also reach parts of the plant above soil
level and invade the plant via its stomata or by penetrating directly if excess water
is left after rainfall or irrigation. The nematode feeds on the cell content by giving
off enzymes that break down the cell walls. When the female nematodes are
sexually mature they produce 200-500 eggs. The stem nematode has a whole
range of host plants. It has the exceptional ability to remain dormant in dry
organic material and even in the absence of a host can survive for years in the
soil.
Cultivate on nematode-free soil and use healthy planting material. Prevent
existing infections from spreading, clean machinery and tools and avoid moving
infected soil. Chicory, carrots, wheat and barley do not allow the stem nematode
to multiply and are not susceptible to damage. Destroy diseased material and do
not leave this debris behind, near or on the field.
ROOT LESION NEMATODE
(NEMATODE)
Pratylenchus penetrans
The root lesion nematode is essentially a worldwide problem. This nematode has
a wide range of host plants. As well as causing direct damage, the nematode also
interacts with other soil borne pathogens.
Symptoms
Typical symptoms exhibited by plants damaged by root lesion nematodes are
stunted growth and a badly developed root zone which lacks the finer root hairs.
Sunken patches can often be noticed on the roots. These lesions are irregular in
shape and in the early stages pale in colour. As the sites age, they become darker.
Root lesion nematodes invade any soil borne parts of the plant. The nematode
directs its attack at young roots. The nematode migrates through the outer tissue
layers from cell to cell by piercing each cell. The content of the cell is converted
into food. 6 to 10 weeks after the initial infection has occurred, many of the
nematodes leave the plant and can be found at many sites in the soil. Some
of them remain behind in the necrotic plant tissue. They are able to withstand
weather influences and can easily survive until the next growing season. After
mating the females deposit their eggs in and on the roots. The transmission
from the first juvenile stage to the second juvenile stage (first molt) occurs within
the egg. After hatching they are able to penetrate the tissue. Depending on the
temperature and the variety of the host plant, the life cycle of the nematode lasts
20-92 days. The ideal temperature for optimal development of the population
ranges from 20 to 30°C. The problems are more severe on lighter types of soil.
Effective control of the root lesion nematode is difficult. As it has a large number
of host plants, crop rotation is only of limited effect. Sugar beet, spinach and
beetroot are poor host plants. Tagetes (marigolds) have a highly antagonistic
effect on Pratylenchus populations. Fallow soil can reduce the infection to some
extent.
Photo: M. Putnam, Oregon State University Plant Clinic.
MAGNESIUM DEFICIENCY
(DEFICIENCY DISORDER)
Symptoms
Magnesium deficiency symptoms are slow plant growth and death of weak
plants. Older leaves become yellow over the entire length of the leaf. Chlorosis
(yellowing) may be followed by browning of the leaf tips and die back.
Scattering or spraying of magnesium sulphate might help.
MANGANESE DEFICIENCY
(DEFICIENCY DISORDER)
Symptoms
Manganese is a catalyst for oxidation and reduction reactions in plant tissue.
This trace element plays an important role in producing chlorophyll. A deficiency
often manifests itself as yellow stripes on the leaf. The plants grow slowly, bulb
development is delayed and they have thick necks. Manganese deficiency is
mainly seen on limy muck soils and during cold conditions. The symptoms are
sometimes confused with Onion Yellow Dwarf Virus. If a manganese deficiency
occurs, it is highly likely that it will still have an impact in subsequent years. This
is particularly the case with crops that are very susceptible for a manganese
deficiency, such as onions, parsley, potatoes, radish, spinach, corn and beetroot.
It is difficult to increase the amount of manganese available in the soil. For this
reason, treat a manganese deficiency using foliar fertilizers. This is the most
effective in a young crop.
HAIL DAMAGE
(PHYSIOLOGICAL)
Symptoms
Damage caused by hail is only visible on the exposed side of the leaves or bulbs.
Round or irregularly shaped spots vary in size corresponding to the size of the
hailstones. The colour of the spots varies from white to yellow. The effects of hail
damage are more significant in a maturing crop. Severe leaf damage results in
onions with delayed bulb formation. These wounds can be a point of entry for
secondary pathogens.
If damage has occurred, spray the onions with a fungicide. This treatment can
also be combined with trace elements, to encourage the wounds to heal and
prevent a further weakening of the plant. If hail damage occurs just before
harvesting and before the foliage has collapsed, it is advisable to flatten the
foliage mechanically to prevent water filling the hollows of the leaves. Preferably
flatten the foliage using a rotating plastic pipe to avoid damage to the leaves and
to reduce the risk of bacterial diseases. Onion bulbs that have been damaged by
hail have a bad or limited storage life
SUN SCALD
(PHYSIOLOGICAL)
Symptoms
High temperatures and bright sunlight during harvest may cause scalding of
exposed onion tissue. The succulent tissue of less mature onions, red onions
or shallots and uncured onions exhibit the injury more likely. Sunscald damage
appears as bleached blotches on the bulb. This tissue becomes soft and slippery
and is a point of entry for secondary pathogens.
If lifting the crops on a hot and sunny day is unavoidable, do not lift the onions
during the hottest hours. Do not leave the lifted onions lying on the field
uncovered for too long. When storing, cool the onions as quickly as possible.
TEMPERATURE STRESS
(PHYSIOLOGICAL)
Symptoms
Regrowth after a period of stagnation due to temperature stress, causing splitting
of the bulb. The basal plate of the onion bursts and becomes corky, in a similar
way to the growth cracks that occur in potatoes. The symptoms are similar to
those of late or excess applications of nitrogen. The split or cracked onions are
unmarketable.
Ensure the soil has a good structure and that the plants emerge uniformly, so that
the onions are subjected to as little stress as possible. Uniform growth will help
prevent the occurrence of cracked onions. If possible, avoid the use of growth
regulators.
TRANSLUCENT AND
LEATHERY SCALES
(PHYSIOLOGICAL
Symptoms
Leathery, water soaked outer scales and skins occur in the field prior to harvest.
If a period of heavy rainfall follows a period of drought during bulb development,
these symptoms might occur. Mechanical bruising, frequent high temperatures
and/or excessive relative humidity may increase leathery scale. Delays between
harvest and storage may also help develop the symptoms.
Prevent rough handling of the onions and damage to the necks. Avoid harvesting
during wet conditions and use artificial drying methods until the neck is
completely dry. Avoid delays between harvest and storage.
CHIMERA
(MUTANT)
Symptoms
Pale green or yellow stripes can be seen on a part of the onion or on a number of
leaves on the same side. These markings are usually visible on the entire length of
the leaf’s surface. This symptom could be interpreted as a virus infection, but is in
fact a localised genetic disorder (mutant).
PD4210 - 06 2012
De Groot en Slot B.V.
Westelijke Randweg 1
1721 CH Broek op Langedijk
T +31 (0)226 33 12 00
E info@degrootenslot.nl
www.degrootenslot.nl
Leading in Allium,
inspired by you.

Major pests and diseases in onion

Transcription

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