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Onion diseases and IPDM
By
M. Mithal Jiskani
Assistant Professor (Plant Pathology)
Faculty of Crop Protection, Sindh Agriculture University
Tandojam
 Onion
is an important bulb vegetable crop, commercially grown
in many countries of the world including Pakistan. Onion
is being used as a raw and given protection to human
beings from sun stroke, normally is consumed green as
well as in mature stages almost by every one (poor as
well as rich people), by different mean. The crop is of
importance in such a mean too that increased production
earns foreign currency through export and in case of
shortages onion is imported by spending many million
rupees.
According to ‘Agricultural Statistics of Pakistan
2004-2005’, onion was cultivated on 55.4 thousand
hectares and produces 633.1 thousand tons during
1987-88, which increased up to 127.8 thousand hectares
and 1764.9 thousand tons during 2004-05, respectively.
The similar source also reported 11.4 to 15.0 tones as
average yield per hectare (Table), which shows
fluctuation in per hectare yield from 9.2 (Sindh) to
20.1 (Balochistan). This fluctuation may be due to
several constraints that affect onion yield adversely in
our country, which may includes the use of low quality
seed, imbalanced fertilizers, and uneven irrigations;
and above all, attack of various insect-pests and
diseases.
In the world, onion is attacked by 66 diseases including
10 bacterial, 38 fungal, 06 nematode, 03 viral and 01
phytoplasmal disease, 01 parasitic plant and 07
miscellaneous diseases and disorders (Mohan and
Schwartz, 2005). In Pakistan, several serious diseases,
such as soil-borne diseases, become widespread and
serious enough to limit production. The diseases
commonly known as downy mildew (Peronospora destructor),
purple blotch (Alternaria porri), grey mold (Botrytis
sp.) and basal/pink rot (Fusarium sp.) etc are the most
destructive diseases, damage the crop and reduced bulb
yield sometimes up to 100%. Several chemical, cultural
and biological methods are used to control onion
diseases, but most of the growers are unaware to that,
similarly many disease aspects needs special attention
by the researchers to work more and extension workers
needs updating themselves to guide the concerned.
Therefore, causes, identification and control of most
important diseases is discussed for the due attention of
onion growers, researchers and extension workers.
Downy Mildew caused by Peronospora destructor is one of
the most serious diseases. Many Allium species including
onion and garlic etc serve as a mean of its alternate
hosts. The disease is characterized as elongated leaf
patches that are slightly paler green or tan and
collapse soon. When wet with dew, grayish violet and
furry growth visible on lesion surface. Under favorable
conditions, this disease rapidly destroys susceptible
tissues. The disease causing fungus produces oospores
that can be soil borne for several years. Pathogen also
over-winters in onion bulbs and as oospores in debris
from diseased foliage. Spores can survive for up to 3
days, are produced at night and dispersed during day.
The spores need 9-16 days between infection and
sporulation. Sporangia can be carried long distances on
air current and are favored by cool temperatures
(43-80OF) and wet weather.
Growers are advised to do not plant infested bulbs.
Rotate with non host crops for at least 3-4 years.
Prompt tillage after harvest to hasten breakdown of
onion tissues. Frequent sprays with fungicide are
effective. Well drained field, lower plant densities and
wind help the disease spread, hence avoid thick sowing
and frequent (heavy) irrigation to avoid prolonged leaf
wetness.
Ahmed and Khan (2002) developed an Integrated Disease
Management Model (IDMM) for control of onion downy
mildew, which included the testing of NPK fertilizer
120:90:60 kg ha*1, plant population 0.5 million plants
ha*1, 8 irrigations/season, fungicide Ridomil @ 250 g
100 L*1 plus Antracol/Dithane M-45 @ 200/300 g 100 L*1
and herbicide Ronstar @ 5 ml L*1. This model reduced the
Area Under Disease Progress Curve (AUDPC) and increased
substantially bulb size (cm) and bulb yield (t ha*1) by
3.7-4.1 and 20.3-28.2%, respectively. The multi location
testing of IDMM proved its superiority over FOPs in
minimizing the disease attack and improving the crop
yield.
Botrytis blight, bulb and neck rot caused by Botrytis
allii, B. squamosa, B. cinerea, is the next important
disease, can damage crop in field and bulbs in field as
well as during storage. The infections originate in the
field but symptoms usually appear after harvest. The
disease first appears as a soft, brown rot at the neck.
When conditions are humid, gray fungal growth develops
on the bulb and between the scales. Black irregularly
shaped fungal resting structures (sclerotia) produced on
the outer neck tissue or the rotten outer scales at
advanced stages of the disease. The neck tissues
collapse shows internal watery decay but the necks are
damaged before they are well dried. Sclerotia of the
fungus frequently found on the neck of onion. The
disease develops best under cool and humid conditions
(50° to 75°F). In the absence of a host, the fungus
survives in the soil and in rotted bulbs as sclerotia
for long periods. The fungus can also be seed borne or
taken into field on infected bulbs planted for seed
production. Airborne spores can penetrate succulent host
tissue without a wound while moisture is present. Necks
that are not cured are extremely susceptible to
infection. The fungus is unable to penetrate well-dried
neck tissue.
Botrytis blight, bulb and neck rot can also be
controlled through long rotations with non-host crops.
Wide row spacing and avoiding excessive and late
applications of nitrogen disfavor disease development.
Following production practices that promote crop
storability e.g. do not irrigate within 10 to 14 days of
lifting onions, allow tops to dry approximately 1 week
before topping, cut roots under bulbs before harvest,
harvest only when the crop is mature, and during dry
weather, avoid damaging necks or bulbs at harvest,
cutting necks 1” above bulbs is preferable to ripping
off tops are also useful. To provide good ventilation
for curing onions before storage and proper storage
conditions: just above freezing and prevent moisture
condensation on bulbs is also helpful to decrease
disease.
Black mold caused by Aspergillus niger produces its
typical characteristic as black mold, growth of the
disease causing fungus, appears as black discoloration
at the neck. Shallow sunken lesions also occur on outer
scales and clusters of black spores below the outer dry
scales. The entire surface of the bulb may turn black
and all scales may be affected in advanced stages. The
bulb may become dry and shrivel, but more often soft-rot
bacteria follow the infection. The spores of this fungus
are very common in the air and soil. Optimum
temperatures for disease development ranged from 82 - 95
OF and moisture also favor disease development.
Avoiding crop injury reduce black mold incidence. Bulbs
should be protected from moisture during harvesting and
shipping. Black mold development can be suppressed by
maintaining transit and storage temperatures below 55
OF.
Blue mold caused by Penicillium sp. appears at
harvesting or in storage. The first symptoms appear as
water soaked areas on the outer surface of the scales. A
blue-green powdery mold will develop on the surface of
the lesion. In advanced stages of the disease the bulb
may become tough and rubbery or a soft-rot may develop
due to action by bacteria that may invade tissue
affected by blue mold. The fungus commonly grows on dead
material and invades onion bulbs through injuries,
bruises and uncured neck tissue. Optimum conditions for
disease development include high relative humidity and
temperatures of 70O– 77OF.
Only avoid wounding the bulbs at harvest and storing
below 42OF temperatures is recommended.
Onion smut caused by Urocystis cepulae, develop typical
smut symptoms on leaves and bulbs. Affected plants are
usually stunted, and often die. Surviving plants often
have poorly developed bulbs or produce bulbs that
deteriorate rapidly in storage. Plant symptoms consist
of dark brown to black, slightly thickened streaks or
blisters on the cotyledonary leaf and other leaves or
scales. Blisters in localized areas of leaves often
rupture to expose black powdery masses of smut spores.
Seed-bed solarization significantly reduced smut
pathogen and improved seedling stand and other
characters including root and shoot lengths, number of
leaves, fresh end dry weighs.
Stemphylium Leaf Blight of Onion caused by Stemphylium
vesicarium (teleomorph: Pleospora allii), initially
produces small, light yellow to brown, and water-soaked
lesions on the leaves and leaf sheaths. As the lesions
expand, they coalesce, causing extensive blighting of
the leaves. Typical lesions are found in higher numbers
on the side of leaves facing the prevailing wind. The
centers of lesions turn brown to tan, then dark olive
brown and finally black as the fungus sporulates.
Sometimes fruiting bodies called perithecia may appear
in infected tissue as small, black, pinhead-like raised
bodies. Infection is usually limited to leaves, and does
not extend down to the scales of the bulb. The fungus
normally invades dead and dying onion tissues. Severe
damage can occur on healthy leaves during warm weather
when leaves are wet for more than 24 hours.
Long-term rotation with unrelated crops may reduce
losses. Also, good field drainage and reduced plant
density may lessen disease severity.
Purple Blotch caused by Alternaria porri, occurs as
water soaked areas followed by white necrotic spots on
foliage. Girdling lesions on the stem and black
concentric rings on purplish lesions also appear. Fungus
requires rain or persistent dew for reproduction and
penetration of plant tissue, can survive in crop debris
Rotation with non-host crops, maintaining good air
movement around foliage is suggested. The growers can
also use proper fungicides.
Fusarium Basal Rot, caused by Fusarium oxysporum f. sp.
Cepae, results yellow and necrotic leaf tip. Affected
plants may wilt; decay of bulbs progressing up from
basal plate, white mycelium on and surrounding basal
plate are also typical characteristics of the problem.
Infection increased by insect injury to roots, stem
plate or bulb and the pathogen is soil borne
Minimum of 4 years crop rotation is proposed.
Translucent Scale disease is cussed by frequent high
temperatures (>90° F) and high relative humidity during
the last 6-8 weeks of growth. The scales are translucent
and resemble freezing injury, become grayish and water
soaked, are brownish inside. Affected scales shrink,
giving bulb slightly irregular surface. Symptoms
continue to progress in storage, delays of 2-4 weeks
between curing and cold storage can increase incidence.
The problem needs research for solution.
Root knot nematode (Meloidogyne hapla, M. incognita, M.
javanica, and M. chitwoodi) feeds on the roots, results
stunting and reduced stand and produces characteristic
galls on roots. Galls induced by M. hapla are generally
small and difficult to see, whereas galls produced by
the other root knot species are larger.
Stubby root nematode (Paratrichodorus sp.) also feed on
roots and plants become stunted.
Lesion nematode (Pratylenchus penetrans) suppresses the
growth and yield.
Growing non host crops for several years is helpful, but
is not usually feasible because nematodes posses wide
host range. However, soil amendments, oilseed cakes,
plant extracts and recommended nematicides help to
decrease nematode population.
Courtesy: The writter is parmanent contributer of
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