2.5.22
EPIDEMIOLOGICAL STUDIES ON THE ROLE OF WEEDS, PLANT DEBRIS AND VECTOR TRANSMISSION IN THE SURVIVAL AND SPREAD OF XANTHOMONAS CAMPESTRIS PV. MANIHOTIS, CAUSAL AGENT OF CASSAVA BACTERIAL BLIGHT

A FANOU1, K VVYDRA1, M ZANDJANAKOU1 and K RUDOLPH2

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

Background and objectives
Cassava is one of the most important staple food crops in Africa. It plays a particular role in food security and serves as source for cash income for subsistence farmers. Because of its long growth cycle, cassava is exposed to many biotic and abiotic constraints. Cassava bacterial blight (CBB) is one of the major diseases and can cause devastating losses. CBB epidemics are recently reported from different areas in West Africa [1]. In order to propose recommendations for the control of the disease, in addition to the use of resistant varieties [2], the role of weeds, plant debris and vector transmission in the survival and spread of Xanthomonas campestris pv. manihotis (Xcm) was studied.

Materials and methods
In a cassava field inoculated with an antibiotic-resistant marker strain, 15 frequent weeds were collected periodically to detect the pathogen, while in the glasshouse, 13 weeds were infiltrated with a marked strain of Xcm. To study the survival of Xcm in plant debris in soil under field conditions, infected cassava leaves were kept on the soil surface, on the soil surface, but covered with soil, or buried at 25 to 30 ;cm. Under glasshouse conditions, infected leaves mixed and not mixed with soil were subjected to different water regimes and dry conditions before quantifying the survival of Xcm. After feeding on cassava plants, artificially infected with a marked strain, Xcm was detected on and in Zonocerus variegatus. The duration of the viability of the pathogen on and in the insects and in faeces, and the pathogenicity of Xcm isolates from faeces were tested.

Results and conclusions
Under field conditions, some weeds such as Solanum nigrum, Tridax procumbens, Talinum triangulare, Mariscus alternifolius, Pupalia lappacea, Brachiaria deflexa and Dactyloctenium aegyptium can lodge Xcm epiphytically for at least for 4 weeks. When Xcm infiltrated the leaves, survival time of the pathogen reached 2 ;months on Vernonia cinerea, Dactyloctenium aegyptium and Brachiaria deflexa. Because weeds grow frequently in and close to cassava fields during the rainy season, it appears that the pathogen can be spread from these weeds in and between cassava fields. Xcm in plant debris kept on soil surface survived at least 2 ;months, but when covered or buried, the survival time was much less. In the glasshouse, the survival reached less than 2 ;months when the debris mixed with soil was moistened three times per week. When unmixed debris was moistened as above, Xcm could survive at least 2 ;months, but not up to 3 ;months. When mixed and unmixed debris were kept in dry conditions, Xcm was isolated after 5 and 4 ;months, respectively. Thus, infected leaves which drop in the field at the end of the rainy season can be an inoculum source for the dry period (4-5 ;months) and infect young plants in the new cropping season. Xcm was detected on the mandibles, legs, in the alimentary canal and the faeces of the insects. The mandibles, legs and the alimentary canal carried few bacteria, while the faeces lodged more bacteria which kept their pathogenicity. Angular leaf spots developed when moistened faeces were maintained on scarified leaves, in holes of leaves and on the adaxial as well as the abaxial surface of intact leaves. The survival time of Xcm on legs and mandibles was shorter than in the alimentary canal and in faeces. We conclude, that Z. ;variegatus can contribute to the transmission of the disease. It was observed that the adaxial surface of cassava leaves can retain faeces during the rainy period.

References
1. Wydra K, Msikita W. 1998. Proceedings 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 9th International Conference Plant Pathogenic Bacteria, Madras 1996, India, in press.