3.7.84
INTERACTIONS BETWEEN COFFEA ARABICAVARIETIES AND ISOLATES OF COLETOTRICHUM KAHAWAE, THE CAUSAL AGENT OF COFFEE BERRY DISEASE
CO OMONDI1, PO AYIECHO2, AW MWANG'OMBE2 and H HINDORF3

1Coffee Research Foundation, P 0 Box 4, Ruiru, Kenya; 2University of Nairobi, P 0 Box 29053, Nairobi, Kenya; 3Institut fur Pflanzenkrankheiten, Nussallee 9, 53115, Bonn, Germany

Background and objectives
Coffee berry disease (CBD), caused by Colletotrichum kahawae Waller & Bridge (1993), is a major constraint to Arabica coffee production in Kenya. The disease attacks all stages of the developing crop including flowers, berries and occasionally leaves, causing heavy losses (1). An Arabica coffee variety, Ruiru 11, developed at the Kenyan Coffee Research Station, Ruiru and released to growers in 1985, is not only resistant to CBD and leaf rust (Hemileia vastatrix), but also combines high yield with good quality and compact growth for high density planting. The years following the release of the variety have been devoted to the improvement of the genetic base of resistance; but there is a problem of possible pathogen variability (2, 3 ). The objectives of this study were therefore to establish the nature of host x isolate interaction and its implication for pathogen adaptation to resistant host varieties as well as to determine the magnitude of genetic diversity in the pathogen population.

Results and conclusions
Pathogenicity tests were performed on 11 Coffea arabica varieties with 90 monoconidial isolates of Colletotrichum kahawae. The main effects of varieties and isolates were highly significant (P<0.01) accounting for 64.7% of the total variation. The variety x isolate effects were also significant (P<0.01) contributing 14.6%. A larger part of the variation in the pathogen. population was therefore due to aggressiveness as reflected by the large main effects of varieties and isolates. The differential effects, although significant, were too small to suggest conclusively that variation for virulence exists. The implications of these results are discussed.

References
1. Mulinge, S.K., 1970.Ann. Appl. Biol. 65, 269-276.
2. Rodrigues Jr. C.J; Varzea, V.M.P.; Hindorf, H. and Medeiros F.F. 1991. J. Phytopathol.131, 205-209. 3. Rodrigues Jr. C.J.; Varzea, V.M. and Medeiros, F.F., 1992. Kenya Coffee 57,1417-1420.