postharvest diseases root and tuber crops

8/19/2019 Postharvest Diseases Root and Tuber Crops

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Postharvest diseases – root and tubercrops

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Most important root and tuber crops

• Cassava• Edible Araceae

• Sweetpotato• Yam


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Introduction• Root and tuber crops are important staple

foods in the tropics– about 700 million people grow and consume rootand tuber crops

– cassava: important in Africa (54% of theproduction), South America, Asia

– yam: most important in Africa– sweetpotato: mostly grown in Asia (89% of the

production)– edible Araceae (cocoyam): of great local

significance (West-Africa, Pacific Islands)


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Cassava (Manihot esculenta)

• Originates in tropical America• Provides food for millions of people in

Africa, Asia and South America• Major producing countries: Brazil, Thailand,Congo, Nigeria, Indonesia, Tanzania,Uganda, India, Paraguay, Mozambique

• Mature roots can be left unharvested for aconsiderable period of time


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• cooking of cassava is necessary to reducecyanide

• Fresh cassava is consumed close to theplace of production• cassava is very perishable, serious

physiological deterioration can occur within24 h of harvest

• deterioration: vascular streaking, followedby microbial deterioration


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cassava - distribution


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Cassava – vascular streaking

• physiological disorder• blue-black discolouration in the ring of vascular

tissue/blue vein; cooking quality and taste are

impaired• roots deteriorate very rapidly after harvest(within 24 h at 35°C)

• control: pruning the aerial parts of the plants 2 or3 weeks before harvest; leave a short stub ofstem attached at harvest; heating the rootsbefore storage; cold storage; high humidity

storage in polyethylene bags; fungicide dip


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cassava – vascular streaking


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Cassava – major diseases (2)

• Fusarium rot– causal agent: Fusarium solani– reported in Nigeria, India, Costa Rica, Colombia– causes a dry rot; white mould growth can be

seen in humid conditions– fungus survives in soil as chlamydospores– infection can take place before or during

harvest

– control: use of postharvest fungicide dip


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Cassava – other diseases

• Aspergillus rot• Bacterial soft rot (Erwinia)

• Mucor and Rhizopus rot• Phytophthora rot• Sclerotium rot• Trichoderma rot


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Edible Araceae (cocoyams)

• Colocasia esculenta (Taro, old cocoyam)– originating in India– grown in West-Africa for 2000 years– major producing countries: Nigeria, China,

Ghana, Japan, Ivory Coast, Papua New Guinea,Burundi, the Philippines, Egypt, Madagascar,Hawaii


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Colocasia esculenta - distribution

http://www.cabicompendium.org/cpc/map.asp?CCODE=CXS_ES


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• Xanthosoma sagittifolium (tannia, macabo,

new cocoyam)– originating in tropical America– introduced in Africa in 19 th century (new

cocoyam)– major producing countries: Nigeria, Cameroon,Ghana, Pacific Islands, Caribbean islands,

tropical America– more robust plant than Colocasia


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cocoyam


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Cocoyams – major diseases (1)

• Cocoyam root rot disease– mainly caused by Pythium myriotylum (especially

Xanthosoma is susceptible)

– infection occurs during the growing season– roots become slimy and rotten; only small unmarketablecorms are produced; infection of normal corms can occurduring harvesting and handling

– Pythium survives in soil as oospores– control: use only healthy planting material, fungicide

treatment, crop rotation, compost


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Cocoyam root rot disease

• Lesions on the root• Dwarfing• Yellowing of the

leaves


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cocoyams – major diseases (2)

• Botryodiplodia rot– caused by Lasiodiplodia theobromae – probably occurs wherever cocoyams are grown– dry rot– fungus is common in tropical soils; infection can

occur in the field or during harvest andhandling

– control: fungicide treatment, curing the corms


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cocoyams – major diseases (3)

• Mal seco– caused by Fusarium solani, Corticium rolfsii

(conidial stage: Sclerotium rolfsii), Rhizoctoniasolani

– soilborne fungi; harvested corms rot in store– control: fungicides, cold storage, use of clean

planting material


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Sweetpotatoes (Ipomoea batatas L.)

• originates in tropical America• major producing countries: China, Uganda,

Nigeria, Indonesia, Vietnam, Japan, India,the Philippines, Brazil, Rwanda, SouthKorea

• usually grown for local consumption• healthy well cured roots can be kept forseveral months


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sweet potato - distribution


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sweetpotato – major diseases (1)• Black rot

– caused by Ceratocystis fimbriata – important in the USA, Peru, Japan, Papua New Guinea,

New Zealand– firm and shallow rot; when root is cooked, flesh tastes

bitter– fungus survives in soil and crop residues aschlamydospores

– infection is facilitated by wounds made by insects and

rodents; infection occurs preharvest– roots may appear healthy at harvest but late rot in storeor during marketing

– Control: crop rotation, clean planting material, careful

handling of harvested roots, storage at 10 to 12°C;postharvest fungicide dip


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sweetpotato – black rot


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sweetpotato – major diseases (2)• Java black rot

– caused by Lasiodiplodia theobromae (= Diplodia gossypina) – most prominent storage disease in the tropics– affected roots become mummified with black pycnidia

– fungus survives in soil; roots are likely to becontaminated at harvest– infection occurs via wounds sustained during harvesting

and handling– temperatures around 30°C favour disease development– Control: fungicide dip before planting; fungicide dip of

roots after harvest and curing at high humidity


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sweetpotato – Java black rot


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sweetpotato – major diseases (3)• Rhizopus rot (soft rot)

– caused by Rhizopus spp.– occurs in all sweetpotato-producing countries– soft and watery rot under humid conditions; strong

development of white mould with sporangia

– Rhizopus is common in soil and atmosphere– harvested roots are likely to be contaminated with

spores; wounds predispose the roots to attack; infectionoccurs postharvest

– infection is especially likely at RH between 75 and 85%– Control: fungicide dip before planting; careful handling of

harvested roots; storage at 13 to 16°C; postharvestfungicide treatments


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sweetpotato – major diseases (4)• Sclerotium rot

– caused by Sclerotium rolfsii (teleomorf =Corticium rolfsii)

– copious white mould growth with sclerotia inhumid conditions– fungus persists in soil or on crop debris; roots

decay in storage– control: deep ploughing to bury the sclerotia;

use of healthy planting material; storage at 10to 12°C


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Sweetpotato – other postharvestproblems

• Diseases– charcoal rot (Macrophomina phaseolina)

(harvest infection)

– Erwinia chrysanthemi soft rot (postharvestinfection)

• Insects– sweetpotato weevil (Cylas formicarius, C.

puncticollis, C. brunneus): field and storage pest


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Yams (Dioscorea spp .)

• Some types of yam– greater yam (Dioscorea alata)– white yam (Dioscorea rotundata)

– yellow yam (Dioscorea cayenensis)– Asiatic yam or lesser yam (Dioscorea esculenta)– Chinese yam (Dioscorea batatas)

• Mainly produced for local consumption• Important in West Africa, Pacific Islands,

Caribbean islands, Brazil


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Yam - distribution


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yam


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Yam – major diseases

• Aspergillus rot– caused by Aspergillus niger and Aspergillus flavus• Blue and green mould rots

– caused by Penicillium spp.• Botryodiplodia rot

– caused by Lasiodiplodia theobromae

• Fusarium rot– caused by Fusarium oxysporum, F. culmorum, F. solani• Rhizopus rot

– caused by Rhizopus stolonifer or R. oryzae


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Root and tuber crops: postharvestcontrol strategies

• Simple, traditional methods– progressive harvesting– heap storage under thatched huts or houses– underground storage in pits, trenches, sand

beds– box storage with absorbent material for aroids

and sweetpotato– pruning techniques– barns, sheds, huts, silos for the storage of yam


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• Modern methods– cold storage

• cassava: best stored at 3°C

• sweetpotato: best stored at 12 to 15°C• yam: best stored at 12 to 16°C• aroids: best stored at 7°C

– freezing• cassava roots can be stored under deep-freeze

conditions in polyethylene bags

– waxing• used for cassava roots; dip in paraffin wax; storagefor up to 1 to 2 months


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– chemical treatment

– Irradiation• UV irradiation of sweet potatoes• gamma irradiation: used for sweet potato and yam• controlled atmospheric storage: used for sweet

potato


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Further reading

• Ravi, V., Aked J., Balagopalan, C. (1996) Review on tropical root andtuber crops. I. Storage methods and quality changes. Criticalreviews in Food Science and Nutrition 36(7): 661-709

• Ravi, V., Aked J. (1996) Review on tropicl root and tuber crops. II.Physiological disorders in freshly stored roots and tubers. CriticalReviews in Food Science and Nutrition 36(7): 711-731

• Ray, R.C., Ravi V. (2005) Post harvest spoilage of sweetpotato intropics and control measures. Critical Reviews in Food Science andNutrition 45: 623-644 (available via Minerva)

postharvest diseases root and tuber crops

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