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Common African Pests and Diseases of Cassava, Yam, Sweet Potato and Cocoyam

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Common African Pests and Diseases of Cassava, Yam, Sweet Potato and Cocoyam Common African Pests and Diseases of Cassava, Yam, Sweet Potato and Cocoyam edited by Robert L. Theberge The International Institute of Tropical Agriculture The Interna ti onal Institute of T rop ical Agriculture (IITA) is an a Ulonomous. nonprofil corporation governed by a IS-memher board o f trustees. lI TA was establis hed in 1967 by the Ford and Rockefeller Foundations. wh ich pro­ vided the initial capital fo r bui ldings and develop­ ment. and the Federal M ilitary Govcrnmenl of Nigeria. who all o tted 1.000 heclares of la nd for a headquarters sit e seven kil ometers north of Ibadan. IlTA seeks to develo p alternatives to shifting cu ltiv<it iol1 that will mainta in the pro ductivi ty of th e land under com inu ous cultivation in the humid and suhhurnid tropics: 10 develop higher yielding pest and disease resista nt varieties of cowpeas, yi:lms and sweet po tatoes worldwide; to increase production in maize, rice. cassava and soybeans in Africa: and to ~ t rc ngth c n national agricultural re search systems by a comprehensiv e training program and collahorative resea rch . II TA is one of 13 nonprofit international agri ( ul­ tural research and training centers supported by the Consult a tive Group for International Agricultural Research (CG IAR). The CGIAR is supported by the Food a nd Agricullure Organization of the United Nations (FAO). the International Bank for Reconstruction and continu(!d on inside bock ('o rer COMMON AFRICAN PESTS AND DISEASES of Cas",.a, Yam, Sweet Potato and Coeoyam COMMON AFRICAN PESTS AND DISEASES of Cassava, Yam, S weet Potato and Cocoyam edited by Robert L. Theberge with contributions bv Fields E. Ca veness - Hans R. Herren Ki- Munseki Lema Peter Neuenschwander Robert L. Theberge John S. Yaninek Foreword by Sang Ki Ha hn International In sti tute of Tropical Agriculture Jbad a n, Nigeria 1985 The Internationa l Institute of Tropical Agriculture P.M.B. 5320 Oyo Road Jbad an , Ni ge ria © lnternational lnstitute o f Tropical Agriculture, 1985 ISBN 97 8- 131-00 1-4 Printed in England by Balding + Mansell Limited ACK~OWLEDGEMENTS The authors "ish to thank Janet Keyser and Chris W. Purisch for their editorial assistance. CONTENTS PaKe Foreword IX Introduction xI Cassava 2 African mosaic di sease 4 Cassava bacterial blight 6 Angular leafspot 10 Cassava anthracnose disease 12 Cassava brown lea fs pot 14 Cassava white leafspot 16 Cassava leafblight 18 Cassava tuber rot 20 Cassava soft rot 20 Dry root rot 24 Root-knot nematodes 26 Cassava green mites 30 Cassava red mites 32 Cassava mealybug 34 Striped mealybug, green mealybug 38 Cassa va scale 40 Elegant grasshoppers 42 Yam 44 Yam mosaic disease 46 Water yam chlorosis 48 Yam bacteri al rot 50 Yam nomF rot 52 Yam. lesio n and root-knot nematodes 54 Yam tuber beetles 58 Stored ya m beetles 60 Yam leaf beet les 62 Sweet Potato 64 Sweet potato virus disease 66 Sweet potato scab 68 Concentric ring spot 70 Root-knot nematodes 72 Sweet potato butterfly 74 Sweet potato weevils 76 Cocoyam 78 Dasheen mosaic virus 80 Bacterial leaf necrosis 82 ScierOlillm blight 84 Phytoph thora leaf bli ght 86 Cladosporium lea f spot 88 Cocoyam root rot blight complex 90 Leptosphaerulina lea fs pot 92 Concentric leafs pot 94 List of Scientific and Common Names 97 Index 103 FOREWORD THE major tropical root crops - cassav,!. yam, sweet potato and cocoyam - are widt!ly grown and mostl y used as subsiste nce staples in tropical and subtropica l Africa. T hey are the major source of energy for \",.ell over 200 mill ion people on the continent. and the leaves. except for ya ms, are oft en used as a green vegetable. These root and tu ber crops are we ll adapted to diverse ,oil and environment al conditions, as well as to complex traditional farming systems. However, because of the primari ly subsistence na ture of these crops, they have hi storically received little attention from scientists and policy-makers. On the African continent, pe~ t s and diseases are the m OSl serious limiting fa ctors in root crop production - limitations tbat wi ll in crease as populations, and areas under cultivation, in­ crease. The Root. and Tuber Improvement Program of the International Insti tute of Tropical Agricultu re (IITA) (Ibadan, Nigeria). was established in 1971 to assist in increasing production of these crops. It s objectives are to develop improved va n eties a nd populations of cassava, sweet potato, yam and cocoyam. ",ith high stable yields. resistance to major diseases and in sect pests and nematodes, wide adapta­ bili ty. good storage cha ra cteristics and high consumer acceptance. The program is also active in traini ng scientists and tech nicians from nati onal root and tuber resea rch programs throughout the humid and subhumid zo nes of Africa and elsewhere. Through these training programs and through the active cooperation and coll abo ration or nati onal program personnel, the results of IITA research are ada pted and transmi tted to the farmers. It is hoped that the results of research and study included in this handbook will be useful to all (hose who labor to increase food crop pro­ duction in the tropics. S. K . H I\H:-< DIRF.f'1 0R. R OOT fi ND T IIBt-.k I).I r>RO\,( M F.S l ' PROGRAM hn:RSATIONAL I "STlfl! T£ 0 1' T ROPICAL. A li RIC UL TURI; INTRODUCTION Common African Pe ,';l .~· and Diseases oI Ca,\'sliva. Yam, Sweet POtalO and Cocoyam describes the most common fungal , bacte ria l, insect and mite problems associated with the cultivation of these four major Iropical roo I crops. While some problems are of major economic importance, oth ers do not present major constraints to produclion. The incidence and the d islribu tion o f Ihe various pests (rlln ga l ~ bact.erial. nematode, insect or mite) associated with these root crops a re based upon lit erature reviews and personal com­ munication. as well as personal observations. T hi s publication is intended as a pictorial guide for growers of cassava, ya m, sweet potato and coco yam in o rder to help identify production constraints ofte n associated with these four root crops. The informa ti o n on the symptoms fo r the various luber crop diseases allows fo r the imple· men tation of appropriate control strategies o nce the problem is correclly identified. The integration of several control measures - cultural. biological, chemical - may prove 10 be most effective . The International Institute of Tropical Agriculture (IITA) places particula r emphasis on developing rool crop varielies tha t are resistant to or tolerant of the majo r economic diseases. In general, crop rotation. sa nitation. resistant varieties. veclor control a nd the use of clean planting ma teri a ls arc sound approaches in conlrolling plant disease. The cost o f a ny conlrol measure, whether used singly or in combination, should be less than the increased value of the crop, to be economicall y feasible, as we ll as readily adaptable by the sma ll -holder farmers who co nst itute th e ITI£ljor producers of food in Africa. C ASS A V A (M anillal escu/en ra) THERE are well over 200 Manilzo l species, of which l\:ilJll ihot escu/ellla Crantz is the most important from a nutriti onal and econom ic point of view. The crop is commonly referred 10 as cassa va, ma ni oc. mad ioca. tapioca and yuca. The plant is a dicotyle don belonging 10 the Euphorbicaceae family. Cassava lypicall y grows as a shrub, and may reach a heighl of up to 4 meters for so me va rieti es. A cassava crop is propagated vegetati vely by stem cuttings, and storage roots average 5- 10 tubers per plant. Cassava is na tive of Latin America and was in troduced to the African continent by Portuguese traders in the late 16th century. Today, cassava is grown on an esti mated 80 million hectares, in 34 Afr ic.an countries. J t is an importa nt crop in subsistence fa rming as it req uires few production skills or inputs, is drought tolerant and produces reason able yields under adve rse conditions. The tubers. which are mainly composed of starch, and lhe leaves. which are consumed as a green vegetable in cert ain regions. contain cyanogeni c glucosides (HCN). Bitter varieties must be processed (by a com­ bin alion of peeling, soaking, fermenting, grind­ ing, dryin g and cooking accord ing to local cus- 10m) befo re either the roots or leaves can be safely con sumed. Afri ca's average cassava tu be r yield of6.0 l!ha is low in comparison to the world average of 8.3 t/ha. T hi s poor yield is pri marily a resu lt of heavy losses due to disease and pest infestati on. 4 C ASSAVA V IR U S DlSEASE: African cassava mosaic disease, cassava mosaic \'irus RANGE : Continental Africa and Southern India. PATHOGEl': Paired or bonded particles averag­ ing 20 x 30 nm belonging to the geminivirus group. VECTOR: Whitefly (Bemisia cabaciGenn.). HOST: Manilroc spp. Other genera have been reported but not confirmed. Symptoms: Differ due tocultivar, viral strain(s) and/or environmental facto rs. Characteristic intermingled patches of normal, light green or yellow to white patches are present on the leaf (photo I). Thcse chlo rotic areas may vary in size from small flecks or spots to the entire leaf being affected , At times, chlorosis may be limited to the basal portion of the le af.
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