4.7.7
DISTRIBUTION OF CASSAVA BACTERIAL BLIGHT IN DIFFERENT ECOZONES IN WEST AFRICA, AND THE INTERACTION OF ENVIRONMENT, SYMPTOM DEVELOPMENT AND GROWTH PARAMETERS OF CASSAVA

K WYDRA1, A FANOU1, JS YANINEK1 and K RUDOLPH2

1InternationaI Institute of Tropical Agriculture, PHMD, Cotonou, Benin; 2Institute of Plant Pathology and Plant Protection, University of Göttingen, Germany

Background and objectives
Cassava is a major staple food for more than 400 million people in Africa, Latin America and Asia and the main carbohydrate supply for the people in many African countries, with sub-Saharan Africa being the biggest cassava producer in the world. In Africa, diseases are the most important biological constraint, and losses attributed to diseases were estimated at 25 million tonnes. Cassava bacterial blight (CBB) epidemics, which caused devastating losses in the 1970s in Africa, are recently reported from different areas in West Africa [1]. Leaf spots, leaf blight, leaf fall and systemic symptoms resulting in the formation of cankers and dieback on the stem are the typical symptoms.

Materials and methods
For diagnostic surveys, 234 sites were visited in the wet season in 1994: rain forest zone (85 sites), transition forest zone (60 sites), wet savanna (Southern Guinea savanna; 44 sites), dry savanna (Northern Guinea savanna; 19 sites), mountain zone (mid altitude and highland; 26 sites). Incidence and severity of CBB were recorded, and data on five other cassava diseases have been published elsewhere [1]. Field experiments were set up in four ecozones, using the susceptible variety Ben 86025 and the 'resistant' variety TMS 30572, planted in a nested arrangement. Field plants were spray-inoculated with a virulent strain of Xanthomons campestris pv. manihotis, incitant of cassava bacterial blight, from the same area. Symptoms were evaluated every 3-4 weeks during the rainy season, and plant growth data collected after 6 and 12 months.

Results and conclusions
Surveys in Ghana, Benin, Nigeria and Cameroon revealed that cassava bacterial blight was present in all ecozones. However, incidence and severity differed in ecozones, indicated by a higher incidence and severity in the transition forest (present in 61% of sites) and the savanna zones (moist savanna and dry savanna, present in 62 and 42% of sites, respectively), and a low incidence in the rainforest. Disease incidence of plants was highest in the wet savanna zone (32%). In Benin, fields with an average severity class of >4 (beginning dieback) were frequently observed in the transition forest and the wet and dry savanna. Disease incidence and severity were variable comparing data of two subsequent years.

Epidemiological studies on disease development on a 'resistant' and a susceptible variety after artificial inoculation in different ecozones revealed a strong effect of environmental factors. In the humid forest zone of southern Nigeria, the formerly resistant variety developed severe symptoms, while the same variety expressed 'field resistance' in the transition forest zone [2]. Symptom development was variable between locations and years. Although in some locations (transition forest, dry savanna zone), tuberous root yield loss due to CBB infection was significant after 6 months, no significant loss was observed after 12 months in both varieties. Yield loss due to CBB varied with ecozones. Our recommendations for screening for resistance are multi-location trials and inoculation with a set of highly virulent strains of different geographic origin.

References
1. Wydra K, Msikita W, 1998. Proceedings of the 6th Triennial Symposium of the International Society for Tropical Root Crops - Africa Branch (ISTRC-AB) (in press).
2. Wydra K, Zinsou V, Fanou A, 1998. Proceedings of the 9th International Conference on Plant Pathogenic Bacteria, Madras, 1996 (in press).