RE-EMERGENCE OF FUSARIUM WILT OF COFFEE IN AFRICA
J FLOOD and D BRAYFORD
CABI Bioscience, U.K. Centre (Egham), Surrey, TW20 9TY, U.K.
Background and objectives
Results and Conclusions
In both countries and at all sites visited, symptoms were typical of a vascular wilt with leaves initially becoming flaccid, followed by desiccation and finally, abscission. Dieback of the branches usually appeared at the top of the tree and spread downwards; the branches may blacken and the tree dies. The bark of the tree becomes hypertrophied with numerous cracks and a characteristic blue-black staining of the xylem is observed under the bark. Fusarium xylarioides was consistently isolated from diseased samples. Also, dark-violet perithecia of the teleomorph Gibberelia xylarioides were observed in the cracks in the bark at the base of the trees during the rainy season; ascospores were easily seen in squashes of the perithecia.
This disease is clearly a serious threat to coffee production in Africa and the causes of its reemergence are under investigation. One possibility is that a new, aggressive genotype of the pathogen may have arisen. Molecular analysis of F. xylarioides isolates have been carried out. Isolates from stem lesions of robusta coffee from the epidemic areas in DRC and Uganda have so far given identical banding patterns using RDNA-IGS RFLP'S, P-tubulin RFLP's and genomic fingerprinting with simple-sequence-repeat primers and M13-core primer (Brayford and Flood, unpublished). These results suggest a low level of genetic diversity amongst epidemic strains. Isolates from other species of coffee (C. arabica, C. exceisa) and parts of Africa (Ivory Coast, Ethiopia) gave different band patterns. These results are surprising for a heterothallic fungus which produces its sexual stage in nature and support the hypothesis that a new, more aggressive strain of the pathogen may have arisen within the wider gene pool of the pathogen population in Africa. Molecular differentiation of isolates from robusta coffee and those from C. arabica and C. exceisa may indicate that host specialization occurs in this pathogen and no C. arabica has been affected in DRC or Uganda during the current disease outbreak.