POST HARVEST DISEASES OF
CARROT
Post harvest diseases of carrot
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Sour rot - Geotrichum candidum
Crown rot - Rhizoctonia solani
Cottony soft rot - Sclerotinia sclerotiarum
Phytophthora root rot - Phytophthora megasperma
Black rot - Alternaria radicina
Root dieback - Pythium debaryanum
Bacterial soft rot - Erwinia carotovora sub sp. caotovora
Crown rot - Rhizoctonia solani
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Damping-off of carrot seedlings and a crown rot later and during
storage
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Field symptoms include premature senescence and death of foliage
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On carrot roots - dark brown sunken lesions or cankers near the
crown or in other parts of the root – cavity spot
Fungus
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Sclerotia - irregular, brown to black and 5mm in dia
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Fungus produces both terminal and intercalary, barrel shaped
chlamydospore
Hyphae of Rhizoctonia solani with right-angled
branching pattern
Mode of spread and survival
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R. solani overwinters in soils as mycelia on plant debris and as dark
brown sclerotia that remain in soil for long periods
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R. solani can spread from plant to plant in closely spaced carrots
when the canopy is fully formed
Epidemiology
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More severe on muck-grown carrots
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Warm temperatures and wet conditions - especially when these
environmental conditions occur near harvest
Control
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Cultural practices that reduce crown injury and enhance soil
drainage and air circulation within the canopy are recommended
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Plant debris from previous crops should be removed before planting
carrots
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Carrots should not follow perennial crops such as alfalfa
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Quintozene – sprayed in the soil before sowing-effective control
Cottony soft rot / white mould - Sclerotinia sclerotiarum
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Foliage - water soaked, dark olive-green lesions associated with
collapsed tissues
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Lesions expand rapidly over the entire leaf, petiole, and rosette with
infected tissues
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Covered by abundant cottony, white mycelium
Lesions of sclerotinia rot advancing
on carrot petioles and the rosette
Collapsed leaves and petioles
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At an advanced stage - affected tissues
exhibit a bleached appearance, and
occasionally an entire plant may collapse
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Large black sclerotia (2 to 20mm) form
externally embedded in the mycelium or
internally, within the pith of the petiole
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Source of inoculum - soil - sclerotia, mycelium or ascospores of fungus
occur
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Temp - 12.9 to 18.5oC
Control
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Storage - Good ventilation
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Use of clean containers, maintenance of temp near 0oC and RH - 95%
Phytophthora root rot or rubbery brown rot
Phytophthora megasperma
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Infected portions of the root become dark brown to black, watersoaked and rubbery in consistency
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Lesions may occur in one or more bands anywhere on the carrot
root
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White mycelia may be present on lesions, which facilitates the
spread of the fungus to adjacent roots
Epidemiology
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Relatively wet soil conditions from excessive rain/irrigation and
temperatures between 70 to 75ºF
Mode of spread and survival
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Soil borne
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Produces zoospores (swimming spores) - easily spread through
water
Management
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Maintaining adequate soil drainage
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Avoid prolonged periods of water saturation
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Storage conditions - temperature at 32º F and RH <95%
Black rot
Alternaria radicina
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Seedling infection results in pre emergence and post emergence
damping-off
Older senescing petioles on mature plants are particularly
susceptible to infection
Provide an avenue for infection of the carrot crown, which appears
as a black ring of decay where the petioles attach to the root (black
crown)
Stored carrots - dry, black, sunken lesions which can decay the
entire root and spread to adjacent carrots
Mode of spread and survival
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Seed borne
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Survive in crop debris or in soil for up to eight years as spores
spread via water and wind
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Infects carrots at any stage of growth during periods of warm
temperatures (greater than 68ºF) and extended leaf wetness
Management
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Cholorothalonil – Seed and soil application
 Fungicides - Iprodione and Strobilurins
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> 8 years rotation with non host crop
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Maintaining proper temperature and humidity (32ºF and 95% RH) in
storage
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Bacillus subtilis & T. viride application
Root dieback
Pythium debaryanum, P. aphanidermatum
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Pythium brown rot and forking
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Root dieback of carrots produces excessively branched or stubbed
roots
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Fungus kills young tap roots after seed germination, reducing root
length and/or stimulating forking
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Forking and stubbing occur, but these symptoms can also be
caused by soil compaction, nematodes
Mode of spread and survival
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Produce sporangia and oospores
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Spores and mycelia are responsible for the spread of the fungus in
the field, which is facilitated by wet soil conditions
Control
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Avoiding excessive watering
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By providing good field drainage
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Planting carrots in deep, friable and well drained soils
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Post emergence fungicides - Mefenoxam - to control damping-off
Bacterial soft rot (Erwinia carotovora sub sp. caotovora)
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Cells become water soaked, middle lamella is destroyed and the
cells collapse
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Soft, watery or slimy consistency
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Rotted tissues – grey to brown, accompanied by foul odour
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In the field, tops of rotted carrots turn yellow and wilt as roots break
down
Bacteria
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Gram –ve, motile with large peritrichous flagella
Mode of survival and spread
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Soil - source of primary inoculum
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Survive in decaying refuse and enter the root principally through
cultivation wounds, harvest bruises, freezing injury, and insect
openings
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Flies – Hylemus cillicrura, H. brassicae – carry the bacteria in their
intestinal tracts
Control
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Careful handling at harvest - minimizes bruising of carrot roots
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Washing - dipping in a solution of sodium hypochlorite (5.25%)
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Storage condition - temp- just above freezing(0oC), RH 90 %
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Crop rotation - alfalfa, beans, beets, corn
Sour rot - Geotrichum candidum
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Soft, watery, colorless decay on carrot roots
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Decayed area - covered with dull, white spores of the pathogen and
a vinegar-like odour may develop
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Fungus - soil inhabitant that infects carrots through wounds
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In storage – warm temp (greater than 32°F) and improperly
ventilated
Fungus
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G. candidum - grows as a series of filamentous cells called hyphae
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Its spores, called conidia or arthrospores, are produced by
fragmentation of the hyphae
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Hyphae and spores are white and appear colorless
Control
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In the field
• good field drainage
• minimizing wounding of carrots
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In storage
• good sanitation (use of new or disinfected storage containers)
• precooling and storing carrots at 32º F is essential
Crater rot- Rhizoctonia carotae
Symptoms
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Band of dark brown necrosis around the crown and horizontal brown
canker-like lesions mostly on the crown and upper roots
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Small pits subsequently developed beneath the lesions, that
enlarged into sunken brown crater lines with a white, flocculent
mycelium produced under high humidity
Typical crater rot caused by R. caratoe on
carrot roots. Note dark brown decay
(arrows) around the crown and upper parts
of the root.
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Bitterness
• Carrots can develop off-flavours or bitterness in cool storage. The cause of these
off-flavours is the production of compounds in carrots called isocoumarins.
Isocoumarins develop when carrots are exposed to ethylene.
• Ethylene is a gas produced naturally by many fruits and vegetables, such as apples,
bananas and tomatoes during ripening.
• Carrots can be stored in sealed plastic bags in domestic refrigerators containing
ethylene producing fruit.
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Phenolic browning
• Phenolic browning (or surface browning) is seen as a browning or discoloration of
the surface of carrot.
• It can develop when carrots are washed and stored in cool rooms for long periods
before packing.
• Abrasion caused by mechanical washing often removes the epidermal layer (outer
skin), exposing the carrot tissue to oxidation of phenolic compounds, which turn
brown, or black in severe cases.
• Phenolic browning can be confused with ‘5 o’clock shadow’ or boron deficiency of
carrots.
• The latter is seen as many small brown spots under the skin, causing the root to
look dull.
Management
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Rapid removal of field heat.
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Application of Dowicide (Ophenylphenol) @ 98 g/L.
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6 – methoxy mellen increase resistance
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Cholorothalonil – Seed and soil application.
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8 years rotation with non host crop.
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Using disease free seed.
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Bacillus subtilis & T.viride application.