4.7.3
INCIDENCE AND DISTRIBUTION OF AFRICAN CASSAVA MOSAIC DISEASE (ACMD) AND ITS WHITEFLY VECTOR, BEMISIA SPP., IN FARMERS' FIELDS ACROSS NIGERIA

LC DEMPSTER1, HW ROSSEL2, NA BOSQUE-PEREZ3 and C OGU3

1PNG University of Technology, PMB, Lae, Papua New Guinea; 2P.v. Vollenhovenlaan 18, 7251 AR Vorden, The Netherlands; 3International Institute of Tropical Agriculture, PMB 5320, Ibadan, Nigeria

Background and objectives
African cassava mosaic disease (ACMD) is caused by a geminivirus (ACMV) which is transmitted by a whitefly vector, Bemisia tabaci. ACMD is prevalent throughout Africa and is considered the most prevalent vector-borne disease of any crop [1]. Cassava is a staple food crop in many regions of the world; in 1990 Nigeria alone produced 16.5% of the world production [2]. Yield losses due to ACMD range from 20 to 95% depending on cassava variety and time of infection. A survey was conducted across Nigeria, the objectives of which were to ascertain the incidence and severity of ACMD in farmers' fields, to assess whitefly population densities, to determine the number and type of cassava varieties grown, and to obtain an insight into farmers' awareness of ACMD across Nigeria.

Materials and methods
In 1994, 63 cassava fields were surveyed across five agro-ecological zones of Nigeria, namely southern humid forest, southern guinea-savanna transition zone, northern guinea-savanna, mid-altitude and sudan-savanna. Varieties of cassava can be differentiated by leaf shape and petiole colour. Twenty plants each of single varieties exhibiting red petioles and green petioles were scored per field. The numbers of whitefly nymphs were counted on leaf number nine of each plant. Samples of leaves showing symptoms of ACMD were collected and analysed by ELISA and sap inoculation to confirm the presence of virus.

Results and conclusions
Overall, the percentage of plants diseased was lowest (36.3%) in the southern humid forest zone, which is the major cassava-growing area. The highest percentage (65.8%) of diseased plants was in the southern guinea-savanna transition zone where cassava is also popular. In contrast, the greatest number of whitefly nymphs was observed in the former zone, with low populations observed in the latter zone (31.2 and 2.3 per leaf, respectively). The lowest population of whitefly nymphs was observed in the sudan-savanna (0.1 per leaf) with an average percentage of diseased plants of 48.1%. On a varietal basis, overall, scored varieties with red petioles hosted a higher percentage of diseased plants (61.0%) than those with green petioles (46.7%), together with a greater average number of nymphs (8.8 and 5.5 per leaf, respectively). Farmers (65%) tended to prefer those varieties with red petioles despite a higher incidence of disease. Disease incidence may be lower in green-petiole varieties as they are often improved varieties recently released by the Ministry of Agriculture. This may also explain why farmers prefer the local red-petiole varieties which they are used to. Overall, farmers were not aware that the yellowing and distortion of leaves was caused by a disease.

With an overall percentage of 54% of the cassava scored exhibiting symptoms of ACMD in this survey, ACMD is indeed prevalent, with most stands apparently becoming diseased through the planting material. Through breeding of popular varieties and release of clean planting material, the incidence of disease may become reduced and yields be increased.

References
1. Geddes AMW, 1990. Bulletin No. 36. Natural Resources Institute, Chatham.
2. FAO, 1991. Production Year Book: 1990, Cassava, 44, 94-95.