Claude Fauquet and Denis Fargette Laboratoire de Phytovirologie, ORSTOM, Abidjan, Ivory Coast

African : Etiology, Epidemiology, and Control

Cassava (Munihor escuknra CrantE; culture or together with maize, baaanas cacassava mos& was first dwribed Euphorbiaceae) is the only species in its (Mmacuminara CoIla), peanuts (Are in 1894 under the name fiaweikrankheit genus that is cultivated as a food crop. chis hypogores L),rice (Oryza sat& L.), and has since bean reported through- In South America, where it originated. beans (Phaseoh vulgar9 L.), or other out Africa and in Madagascar, Zanzi- cassava was domesticated 2,0004,000 crops. bar. Seychelles, India, and Java. In East years B.c.,yet only recently has it become Cassava is cultivated throughout the Africa, the disease was not reported to distributed worldwide. The Portuguese year between the latitudes 30° north and cause serious 10- until the 1920s. In began importing cassava into the Gulf south, up to a maximum altitude of West Africa, it was fmt recorded in the of Guinea in Africa in the 16th century. about 2,000 m. It tolerates drought but coatid areasr of Nigeria, Sierra Leone, In the 18th century, they introduced it grows kst where annual Wallreahes and Ghana in 1929 and had spread to the east coast of Africa and the Indian 1,000-2,000 mm. Cassava grows well on northward by 1945. Oman islands of Madagascar, Rhnion, many soil types, with the exception of Responses to au international qw- and Zanzibar. Portuguese ships probably hydromorphic soils, which are unsuit- tionnaire sent to all cassava-producing carried cassava to India and Sri Lanka able. It is propagated vegetatively by African countries in 1987 (10) showed after the mid-18th century. Cassava at stem cuttings, and the growth cycle ma- that ACMV is present wherever cassava first was little appreciated, but since the erdy ranges bttween 10 and 30 months. is grown on the continent (Table l), with 19th century it has extended rapidly Numerous constraints affect produe the wettest regions the mosi seriously across Africa (Fig. 1) and is now grown tivity of cassava in Africa. The most affected. The vim infects dl cassava in 39 countries. widely distributed pathogen is African cultivars, dthough susceptibility varies Cassava is the third largest source of cassava mosaic virus (ACMV),dis- greatly. Incidence of ACMV infection is carbohydrates for human food in the seminated by its vector, Bmisia often extremely high; a survey of 20 world (estimated annual yield is 136 rabaci Genn., which is pmnt in all cm farms in Ghana revealed an average 96% million tons), and Africa is the lrugest sava-growing areas, and by man, in in- of plants infected. Similarly, the inci- center of production (57 million tom fected cassava stem cuttings (Fig. 2). dence exceeded 80% in some districts of were grown on 7.5 million hectm in WbiMics also infmt cotton (Gossypium Kenya and was host 100% in a few 1985). Cassava is produced for industrial hirsurum L.), tobacco (Nicotianu taba- small holdings. In ivory Coast, the id- purposes in Brazil and as an export crop cum L.), and sweet potato (&omuea dence is uearly 100% in every cassava in Thailand, but in Africa it is grown batatas (L.) Lam.), but cassava k the only plantation. Thus, ACMV is widtspread primarily for local food consumption. In cultivated host affected by ACMV. Other and destructive in the humid lowland fact, cassava is the most important food important diseases in Africa are vascu- amas- of Africa+ crop grown on the African continent, ex- lar bacteriosis cawed by Xmthornonas ceeding yam (Dioscorea spp.), 24 million campstrh (Pammel) Downpv. muni- Symptomatdog y tons; maize (2hmays L.). 12 don kotb (&fit and Bondar) Dye; anthxac- The molt visible symptom of the dL tons; and pearl millet (Pennisetumghw nose caused by Col&totrfckum gfm- ease is the expremion of the characteristic cum (L.) R. Sr.), 9 million tons. Pro- sporioirdes f. q.mmrfhozis Ham-;and, leaf mosaic, and young plants are more duction per hectare averages 5-10 tons, to a hsm extent, cercosporiol caused severely affected than old ones. Symp- but on the basis of yields at research hy Cercosporidium hemingsii (Allesck) toms range from barely perceptible mo- stations. the potential exceeds 80 tons. Deighton. The most signifl-t pests me saic to stunting of the plant and extreme In most Africm countries, cassava is mealybug (Phenacoccus manihoti Mat.- reduction of the leaf blades (Fig. 3). The grown primarily by traditional farmers, Ferr.), peen spider mite (Mononychelhu =verity of symptoms is readily quanti- but there are some industria1 plantations spp.), and the variegated grasshopper fied on a scale of 1-5, where 0 = no in Liberia, Nigeria, and Togo. African (Zonocem variegafus L.). symptoms and 5 = leaves reduced to farmers usually grow cassava in mono- veins {Fig. 4). Severity of symptoms Regional Importance of ACMV varies with the cdtivar and increases with Pwnt addrcss of first uuuthoc Washington Uni- ACMV is wnsidmd to be of African plant age until about 60 days after versity. Department of Biology, Box 1137, St. Louis, origin and is unknown in South America. planting. Thereorfter, symptoms are more MQ 63130. Present adPnss of second author: Scottish Crop RewhInstitu~c,Virology Division. However, similar symptoms are induced moderate or lessen or do not develop, DDZ-SDA Dundte, Seatland. by the unrelated South American virus, depending on the cultivw, climatic eon- cassava common mosaic vim, a potex- ditions, and season. Symptoms on fully 0 198a Tha Amerlurn Phytopthdoglml SoeIety virus having roddaped particles. Afri- expanded leaves do not change, however. 404 Plant Oisease/Vol. 74 No. 6