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African Cassava Mosaic Virus Is Transmitted by (A) the Adult Whitefly (Bemisia Fabaci) and (B) Infected Stem Cuttings of Cassava

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African Cassava Mosaic Virus Is Transmitted by (A) the Adult Whitefly (Bemisia Fabaci) and (B) Infected Stem Cuttings of Cassava Claude Fauquet and Denis Fargette 1-3.” P< Laboratoire de Phytovirologie, ORSTOM, Abidjan, Ivory Coast culture or together with maize, bananas can cassava mosaic was first described (Musa acuminata Colla), peanuts (Ara- in 1894under the name Krauselkrankheit chis hypogaea L.), rice (Oryza sativa L.), and has since been reported through- beans (Phaseolus vulgaris L.), or other out Africa and in Madagascar, Zanzi- crops. bar, Seychelles, India, and Java. In East Cassava is cultivated throughout the Africa, the disease was not reported to year between the latitudes 30’ north and cause serious losses until the 1920s. In south, up to a maximum altitude of West Africa, it was first recorded in the about 2,000 m. It tolerates drought but coastal areas of Nigeria, Sierra Leone, grows best where annual rainfall reaches and Ghana in 1929 and had spread 1,000-2,000 mm. Cassava grows well on northward by 1945. many soil types, with the exception of Responses to an international ques- hydromorphic soils, \Irhich are unsuit- tionnaire sent to all cassava-producing able. It is propagated vegetatively by African countries in 1987 (IO) showed stem cuttings, and the growth cycle gen- that ACMV is present wherever cassava erally ranges between 10 and 30 months. is grown on the continent (Table l), with Numerous constraints affect produc- the wettest regions the most seriously tivity of cassava in Africa. The most affected. The virus infects all cassava widely distributed pathogen is African cultivars, although susceptibility varies cassava mosaic virus (ACMV), dis- greatly. Incidence of ACMV infection is seminated by its whitefly vector, Bemisia often extremely high; a survey of 20 tabaci Genn., which is present in all cas- farms in Ghana revealed an average 96% sava-growing areas, and by man, in in- of plants infected. Similarly, the inci- center pf production (57 million tons fected cassava stem cuttings (Fig. 2). dence exceeded 80% in some districts of il were gqown on 7.5 million hectares in Whiteflies also infest cotton (Gossypium Kenya and was almost 100% in a few - 198Y). Gassava is produced for industrial hirsutum L.), tobacco (Nicotiana taba- small holdings. In Ivory Coast, the inci- - purpos{s in Brazil and as an export crop cum L.), and sweet potato (Ipomoea dence is nearly 100% in every cassava j 0- SZEZZ batatas (L.) Lam.), but cassava is the only plantation. Thus, ACMV is widespread U- m in Thailand, but in Africa it is grown --~ primarily for local food consumption. In cultivated host affected by ACMV. Other and destructive in the humid lowland 1 j-coc- IE-m-N nl fact, cassava is the most important food important diseases in Africa are vascu- areas of Africa. 3- lar bacteriosis caused by Xanthomonas g-o crop gròwn on the African continent, ex- 10-o ceeding yam (Dioscoreaspp.), 24 million campestris (Pammel) Dowson pv. mani- Symptomatology 1 ;= o tons; maize (zea mays L.), 12 million hotis (Berthet and Bondar) Dye; anthrac- The most visible symptom of the dis- tons; and pearl millet (Pennisetum glau- nose caused by Colletotrichuni gloeo- I IL___- ease is the expression of the characteristic 1 - cum (LI) R. Br.), 9 million tons. Pro- sporioides f. sp. nianihotis Henn.; and, leaf mosaic, and young plants are more duction per hectare averages 5-10 tons, to a lesser extent, cercosporiosis caused severely affected than old ones. Symp- but on /the basis of yields at research by Cercosporidium henningsii (Allesch.) toms range from barely perceptible mo- Deighton. The most significant pests are saic to stunting of the plant and extreme mealybug (Phenacoccus manihoti Mat.- reduction of the leaf blades (Fig. 3). The Ferr.), green spider mite (Mononychellus severity of symptoms is readily quanti- spp.), and the variegated grasshopper fied on a scale of 1-5, where O = no (Zonocerus variegatus L.). symptoms and 5 = leaves reduced .to veins (Fig. 4). Severity of symptoms Regional Importance of ACMV varies with the cultivar and increases with Present address of first author: Washington Uni- ACMV is considered 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. Thereafter, symptoms are more MO 63130. Present address of second author: Scottish Crop Research Institute, Virology Division, However, similar symptoms are induced moderate or lessen or do not develop, DD2-5DA Dundee, Scotland. by the unrelated South American virus, depending on the cultivar, climatic con- cassava common mosaic virus, a potex- ditions, and season. Symptoms on fully ~ @ 1990The American Phytopathological Society virus having rod-shaped particles. Afri- expanded leaves do not change, however. No. 404 Plant Disease/Vol. 74 6 .". iI a c I . 1 Fig. 1. Principal routes of dissemination of cassava from South America to Africa and India, and distribution of cassava cultivation in Africa. The hatched lines mark the northern and southern limits of cassava production, and the dates are the years African cassava mosaic disease was first described in each country. ent countries to range from 20 to 95% in 1969 was 29 t/ ha from those with mild Yield Losses (Table 1). The estimated average yield symptoms and only 9 t/ha from those ACMV is arguably the most important loss induced by ACMV is 50%. with severe symptoms. Differences in disease of virus etiology in Africa, but As intensity of symptoms increases, yield due to ACMV were greater than total losses are extremely difficult to yield of cassava declines dramatically those related to differences among culti- estimate. Yield losses with individual (Fig. 5). The mean yield of tubers from vars. Even cultivars considered to be re- cultivars have been reported from differ- a collection of cultivars in Ivory Coast sistant may suffer losses of 24-78% (1,6). Fig. 2. African cassava mosaic virus is transmitted by (A) the adult whitefly (Bemisia fabaci) and (B) infected stem cuttings of cassava. Plant Disease/June 1990 405 With the moderately susceptible culti- euphorbiaceous species, Jatropha multi- the name Indian cassava mosaic virus var CB, yield losses range from O to 77%, fid L. Two other species, Hewittia sub- (ICMV) is now used for group C strains. depending on the mode and time of lobata (L.f.) Kuntze (Convolvulaceae) Mechanical transmission of ACMV infection (Fig. 6) and on environmental and Laportea aestuans (L.) Chew (Urti- was first reported in 1978. Purified prep- factors. Yield loss is greater (55-77%) in caceae), are suspected to be natural hosts arations from cassava plants in western cassava plants grown from infected for ACMV in Kenya and West Africa, Kenya contained a geminivirus, but Bock cuttings than in plants infected later by but the virus has not been transmitted et al (2) were unable to obtain this virus whiteflies (35-60%), even when infection from them back to cassava. from all infected plants. The etiology of occurs early. Infection that occurs by The geminate virus particles (Fig. 7) the disease therefore remained doubtful, means of vector 150 days or more after measure about 30 X 20 nm, and the coat and the geminivirus was named cassava planting has little effect on production protein has a molecular weight of about latent virus (CLV) (2) until it became (8). A virus-infected cassava plant remote 30,000. The particles contain one mol- apparent that the isolation problems from healthy cassava may produce 70% ecule of circular single-stranded DNA were due to differences among strains of less than its neighbors, but when all (Mr about 0.92 X lo6), and the genome the virus. CLV was shown to cause mo- plants in a plantation are infected, the consists of two circular molecules of saic and is correctly named ACMV (4). loss may be only 33% because the smaller similar size (15). In leaf tissue, the virus ACMV is mechanically transmissible infected plants compete less with one particles accumulate mainly in the nuclei from cassava to several solanaceous another than do healthy plants. of phloem parenchyma and of cortical plants, including species of Nicotiana On the basis of available data, the total and epidermal cells. and Datura, but successful transmission reduction of cassava yield in Africa in The type strain of ACMV (ACMV-T), back to cassava is difficult and is feasible plants derived from diseased cuttings is isolated in western Kenya, is serolog- only with very susceptible cassava at least 50%, or 50 million metric tons, ically related to all other whitefly-trans- cultivars. N. benthamiana Domin is the per year and may be equivalent to $2 mitted geminiviruses. ACMV isolates best source of virus particles for puri- billion (U.S.). from mosaic-affected cassava collected fication; chlorotic local lesions are frqm different parts of Africa and from followed by severe systemic leaf curling. India are partitioned into three groups D.stramonium L. can be used for local The Causal Agent on the basis of serology and DNA lesion tests; when it is inoculated with The viral etiology of African cassava hybridization (12). Group A includes some isolates, chlorotic and necrotic mosaic was first proposed by Storey in strains from Angola, Ivory Coast, Ni- local lesions are followed by systemic 1936 (17), who demonstrated in Tanzania geria, Congo, and western Kenya and veinbanding and leaf distortion. Virus that the disease was transmissible and also defective strains that do not produce infectivity in sap is unstable, being lost inferred that a virus was responsible. He virus particles; group B strains are from in a few days at room temperature or also studied virus transmission by B. coastal Kenya, Madagascar, and Ma- in 10 minutes at 55 C (3). tubaci (16,17). The establishment of lawi; and group C strains are from India transmission of ACMV in a persistent and Sri Lanka.
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