POST HARVEST DISEASES OF CARROT

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POST HARVEST DISEASES OF CARROT
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Post harvest diseases of carrot

• 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

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Crown rot - Rhizoctonia solani

• Damping-off of carrot seedlings and a crown rot
  later and during storage
• Field symptoms include premature senescence and
  death of foliage
• On carrot roots - dark brown sunken lesions or
  cankers near the crown or in other parts of the
  root cavity spot

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• Fungus
• Sclerotia - irregular, brown to black and 5mm in
  dia
• Fungus produces both terminal and intercalary,
  barrel shaped chlamydospore

Hyphae of Rhizoctonia solani with right-angled
branching pattern
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• Mode of spread and survival
• R. solani overwinters in soils as mycelia on
  plant debris and as dark brown sclerotia that
  remain in soil for long periods
• R. solani can spread from plant to plant in
  closely spaced carrots when the canopy is fully
  formed
• Epidemiology
• More severe on muck-grown carrots
• Warm temperatures and wet conditions - especially
  when these environmental conditions occur near
  harvest

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• Control
• Cultural practices that reduce crown injury and
  enhance soil drainage and air circulation within
  the canopy are recommended
• Plant debris from previous crops should be
  removed before planting carrots
• Carrots should not follow perennial crops such as
  alfalfa
• Quintozene sprayed in the soil before
  sowing-effective control

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Cottony soft rot / white mould - Sclerotinia
sclerotiarum

• Foliage - water soaked, dark olive-green lesions
  associated with collapsed tissues
• Lesions expand rapidly over the entire leaf,
  petiole, and rosette with infected tissues
• Covered by abundant cottony, white mycelium

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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
• 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
• Temp - 12.9 to 18.5oC
• Control
• Storage - Good ventilation
• Use of clean containers, maintenance of temp near
  0oC and RH - 95

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Phytophthora root rot or rubbery brown
rotPhytophthora megasperma

• Infected portions of the root become dark brown
  to black, water-soaked and rubbery in consistency
• Lesions may occur in one or more bands anywhere
  on the carrot root
• White mycelia may be present on lesions, which
  facilitates the spread of the fungus to adjacent
  roots

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• Epidemiology
• Relatively wet soil conditions from excessive
  rain/irrigation and temperatures between 70 to
  75ºF
• Mode of spread and survival
• Soil borne
• Produces zoospores (swimming spores) - easily
  spread through water
• Management
• Maintaining adequate soil drainage
• Avoid prolonged periods of water saturation
• Storage conditions - temperature at 32º F and RH
  lt95

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Black rot Alternaria radicina

• 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

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• Mode of spread and survival
• Seed borne
• Survive in crop debris or in soil for up to eight
  years as spores spread via water and wind
• Infects carrots at any stage of growth during
  periods of warm temperatures (greater than 68ºF)
  and extended leaf wetness
• Management
• Cholorothalonil Seed and soil application
• Fungicides - Iprodione and Strobilurins
• gt 8 years rotation with non host crop
• Maintaining proper temperature and humidity (32ºF
  and 95 RH) in storage
• Bacillus subtilis T. viride application

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Root dieback Pythium debaryanum, P.
aphanidermatum

• Pythium brown rot and forking
• Root dieback of carrots produces excessively
  branched or stubbed roots
• Fungus kills young tap roots after seed
  germination, reducing root length and/or
  stimulating forking
• Forking and stubbing occur, but these symptoms
  can also be caused by soil compaction, nematodes

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• Mode of spread and survival
• Produce sporangia and oospores
• Spores and mycelia are responsible for the spread
  of the fungus in the field, which is facilitated
  by wet soil conditions
• Control
• Avoiding excessive watering
• By providing good field drainage
• Planting carrots in deep, friable and well
  drained soils
• Post emergence fungicides - Mefenoxam - to
  control damping-off

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Bacterial soft rot (Erwinia carotovora sub sp.
caotovora)

• Cells become water soaked, middle lamella is
  destroyed and the cells collapse
• Soft, watery or slimy consistency
• Rotted tissues grey to brown, accompanied by
  foul odour
• In the field, tops of rotted carrots turn yellow
  and wilt as roots break down

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• Bacteria
• Gram ve, motile with large peritrichous flagella
• Mode of survival and spread
• Soil - source of primary inoculum
• Survive in decaying refuse and enter the root
  principally through cultivation wounds, harvest
  bruises, freezing injury, and insect openings
• Flies Hylemus cillicrura, H. brassicae carry
  the bacteria in their intestinal tracts
• Control
• Careful handling at harvest - minimizes bruising
  of carrot roots
• Washing - dipping in a solution of sodium
  hypochlorite (5.25)
• Storage condition - temp- just above
  freezing(0oC), RH 90
• Crop rotation - alfalfa, beans, beets, corn

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Sour rot - Geotrichum candidum

• Soft, watery, colorless decay on carrot roots
• Decayed area - covered with dull, white spores of
  the pathogen and a vinegar-like odour may develop
• Fungus - soil inhabitant that infects carrots
  through wounds
• In storage warm temp (greater than 32F) and
  improperly ventilated

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• Fungus
• G. candidum - grows as a series of filamentous
  cells called hyphae
• Its spores, called conidia or arthrospores, are
  produced by fragmentation of the hyphae
• Hyphae and spores are white and appear colorless

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• Control
• In the field
• good field drainage
• minimizing wounding of carrots
• In storage
• good sanitation (use of new or disinfected
  storage containers)
• precooling and storing carrots at 32º F is
  essential

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Crater rot- Rhizoctonia carotae

• Symptoms
• Band of dark brown necrosis around the crown and
  horizontal brown canker-like lesions mostly on
  the crown and upper roots
• 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.
• 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 oclock
  shadow or boron deficiency of carrots.
• The latter is seen as many small brown spots
  under the skin, causing the root to look dull.

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• Management
• Rapid removal of field heat.
• Application of  Dowicide (Ophenylphenol) _at_ 98
  g/L.
• 6 methoxy mellen increase resistance
• Cholorothalonil Seed and soil application.
• 8 years rotation with non host crop.
• Using disease free seed.
• Bacillus subtilis  T.viride application.