POST HARVEST DISEASES OF CARROT

1. POST HARVEST DISEASES OF CARROT

2. Post harvest diseases of carrot Sour rot - Geotrichumcandidum Crown rot - Rhizoctoniasolani Cottony soft rot - Sclerotiniasclerotiarum Phytophthora root rot - Phytophthoramegasperma Black rot - Alternariaradicina Root dieback - Pythiumdebaryanum Bacterial soft rot - Erwiniacarotovora sub sp. caotovora

3. Crown rot - Rhizoctoniasolani • 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

4. Hyphae of Rhizoctonia solani with right-angled branching pattern Fungus • Sclerotia - irregular, brown to black and 5mm in dia • Fungus produces both terminal and intercalary, barrel shaped chlamydospore

5. 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

6. 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

7. Cottony soft rot / white mould - Sclerotiniasclerotiarum 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

8. Lesions of sclerotinia rot advancing on carrot petioles and the rosette Collapsed leaves and petioles

9. 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

10. 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%

11. Phytophthora root rot or rubbery brown rotPhytophthoramegasperma • 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

12. 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%

13. Black rot Alternariaradicina • 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

14. 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. virideapplication

15. Root dieback Pythiumdebaryanum, 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

16. 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

17. Bacterial soft rot (Erwiniacarotovora 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

18. 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 – Hylemuscillicrura, 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

19. Sour rot - Geotrichumcandidum 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

20. 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

21. 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

22. Crater rot-Rhizoctoniacarotae Typical crater rot caused by R. caratoe on carrot roots. Note dark brown decay (arrows) around the crown and upper parts of the root. 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

23. 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.

24. Management Rapid removal of field heat. Application of  Dowicide (Ophenylphenol) @ 98 g/L. 6 – methoxymellen increase resistance Cholorothalonil – Seed and soil application. 8 years rotation with non host crop. Using disease free seed. Bacillus subtilis&T.virideapplication.