Transcript Carrot
POST HARVEST DISEASES OF
CARROT
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
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
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
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
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
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
Lesions of sclerotinia rot advancing
on carrot petioles and the rosette
Collapsed leaves and petioles
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
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%
Phytophthora root rot or rubbery brown rot
Phytophthora megasperma
Infected portions of the root become dark brown to black, watersoaked 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
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 <95%
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
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
> 8 years rotation with non host crop
Maintaining proper temperature and humidity (32ºF and 95% RH) in
storage
Bacillus subtilis & T. viride application
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
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
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
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
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 32°F) and improperly
ventilated
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
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
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.
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 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
Rapid removal of field heat.
Application of Dowicide (Ophenylphenol) @ 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.