ONION
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Neck Rot - Botrytis allii
Blue Mould Rot - Penicillium spp
White rot - Sclerotium cepivorum
Bulb rot/ basal rot - Fusarium oxysporum f.sp. cepae
Macrophomina rot - Macrophomina phaseoli
Purple blotch - Alternaria porri
Black stalk rot -Stemphylium botryosum
Black mould - Aspergillus niger
Neck Rot - Botrytis allii
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Latent disease - although infection takes place in the field
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Softening of scales which take on a water soaked appearance
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Under moist conditions - greyish sporulating mycelial mat develops
on the surface of the scales
Mode of spread and survival
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Fungus persists saprophytically on dead onion tissue, on humus in
soil and as sclerotia near the surface of previous onion crop cells
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Sclerotia germinate in moist weather and produce conidia that are
disseminated by air currents
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When conidia land on the onion necks that are cut or wounded they
can geminate, penetrate and kill onion tissue
Control
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Dusting the seed with benomyl at 1g/kg of seed
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Avoid excessive and late (after mid-July) applications of nitrogen
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Rapid and thorough drying directly after harvest
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Provide good ventilation for curing onions before storage
Blue Mould Rot - Penicillium spp.
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Initial symptoms - water soaked areas on the outer surface of scales
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Later, a green to blue green, powdery mould may develop on the
surface of the lesions
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Infected areas of fleshy scales are tan or grey when cut
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In advanced stages, infected bulbs may disintegrate into a watery
rot
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Penicillium digitatum, P. oxalicum, P. expansum, P. luteum
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Produces enormous number of spores on a broom like conidiophore
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Saprophytes on plant debris and senescent plant tissue
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Invasion - through wounds, bruises, or uncured neck tissue
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Optimum conditions include moderate temperatures (21° to 25°C)
and high relative humidity
Management
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Avoid wounds and insect damage to bulbs
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Harvest and handle onion bulbs with a minimum of bruising or
wounding
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Cure the bulbs so the necks are dry
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Store bulbs at temperatures of 41°F (5°C) or less with low relative
humidity
White rot / mouldy nose/ Dry rot
(Sclerotium cepivorum)
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Leaves - yellow and dieback
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Roots - rotten & base of the bulb covered with a watery or grey
fungal growth
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Numerous small black spherical sclerotia are produced
Mode of spread and survival
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Sclerotia persists in soil for eight years
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Pathogen has no direct sporulation stage and it is not air borne
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Sclerotia are transported from field to field by flood water
Management
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Long rotation of 8 to 10 yrs
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Good drainage
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Use of healthy planting materials
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Destruction of wild onion and leeks
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Seed dressings-benomyl/carbendazim
Bulb rot/ basal rot
(Fusarium oxysporum f.sp. cepae)
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Occur in patches
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Leaves - turn yellow and then dry up slowly
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Entire plant shows complete drying of the foliage
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Bulb - shows soft rotting and the roots get rotted
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Whitish mould growth on the scale
Fungus
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Produces many chlamydospores which are thick walled resting
spores
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Microconidia – one celled and thin walled
Macroconidia
Fusarium sp
Mode of spread and survival
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Soil borne
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Infection occurs through root, either directly or through wounds
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Soil insects are common carriers
Epidemiology
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High temp and low level of soil moisture - favourable for high
disease incidence
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Temp- 28 to 32oc
Management
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Field sanitation - destruction of infected plant debris
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Soil drenching - COC-0.25%
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Pre planting treatment of bulbs - benomyl 15%+ mancozeb 60%
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Local Bellary, Poona Red Globe, Patna Red, White Large - resistant
varieties
Black stalk rot (Stemphylium botryosum)
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Affects garlic and leek
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Attacks only diseased, injured bulbs and aging tissue
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Damage by weakening the seed stalks when they are almost ripe
causing them to break over
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Spoil the appearance of the bulb with a sooty mould before they are
marketed
The fungus Stemphylium
botryosum growing and sporulating
on top of a downy mildew lesion
Fungus
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Producing short clavate to cylindrical asci in brown pseudothecia
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Ascospores yellow to yellowish brown, oval to ellipsoid, 26-50 x 1020 um with 1-5 vertical and 7 transverse septa
Conidia - Stemphylium botryosum
Control
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Harvest without causing any bruises
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Bulbs should be dried properly before storage
Purple blotch/scald disease
Alternaria porri
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Leaves - whitish minute dots on leaves with irregular chlorotic areas
on tip portion
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Circular to oblong concentric black velvety rings appear in chlorotic
area
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Lesions develop towards the base of the leaves
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Leaves die from the tip downwards, break at the point of infection
and hang down
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Bulbs – semi watery rot, shrinkage of the fleshy bulb scales and
desiccation followed by these scales becoming dry and papery
Fungus
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Mycelium - branched, coloured and septate
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Conidiophores - singly or in groups, septate, pale to mid brown
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Conidia - solitary, straight or curved
Conidia
Conidium
Mode of spread and survival
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Carried through seed bulbs collected from infected field
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Spreads mainly through air borne spores
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Pathogen enters through stomata or wounds
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Survives in plant debris for 8 months
Epidemiology
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Temp - 21 to 30oc
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RH - 90 %
Management
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Seed treatment - thiram @ 2.5g/kg of seed
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Three sprays of,
• Copper oxychloride - 0.25%
• Chlorothalanil - 0.2%
• Zineb - 0.2%
• Mancozeb - 0.2%
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Varieties - New Selection, Red Creole - resistant
Black mould - Aspergillus niger
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Masses of black powdery spores on both exterior and between the
scales of the bulbs
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Especially along the vascular strands of the veins
Fungus
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Mycelium - branched, septate thick walled foot cells differentiate and
give rise to a single conidiophore
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Conidiophore - globose on which brown sterigmata are formed
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Vesicle, sterigmata, conidia make up the black head - characteristic
of the fungus
Control
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Rapid and thorough curing
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Storage - good ventilation
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Temp - just above 0oc
Macrophomina rot
Macrophomina phaseoli
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Rotting of onion in storage
Diseased bulbs become ashy
Close examination shows – dark charcoal coloured blemishes on
one or two scales below the outer papery scale
Many pin head size sclerotia- embedded in affected tissues
Control
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Dipping bulbs in formalin 0.03% or boric acid 2.0%