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

Sclerotia - irregular, brown to black and 5mm in dia 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, 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

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.