Expl Agric. (2012), volume 48 (1), pp. 85–98 C  Cambridge University Press 2011 doi:10.1017/S0014479711000883 RESISTANCE TO DIEBACK DISEASE CAUSED BY FUSARIUM AND LASIODIPLODIA SPECIES IN CACAO ( THEOBROMA CACAO L.) GENOTYPES By RICHARD ADU-ACHEAMPONG†, SIMON ARCHER and SIMON LEATHER Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berkshire SL5 7PY, UK (Accepted 24 August 2011; First published online 26 September 2011) SUMMARY Fusarium and Lasiodiplodia species invade feeding lesions caused by mirid bugs (Distantiella theobroma [Dist.] and Sahlbergella singularis Hagl.) and inflict serious damage and yield loss to susceptible cacao (Theobroma cacao L.) varieties in West Africa. As it is the fungal invasion rather than the physical feeding injury by mirids that cause dieback and tree death in cacao, a dieback resistance strategy in cacao crop must take into account resistance to these causal agents. Twenty-nine and 15 cacao genotypes were screened in the laboratory and the greenhouse, respectively, for resistance to isolates of Fusarium decemcellulare and Lasiodiplodia theobromae at Imperial College London’s Biological Sciences Campus, UK. Resistance was assessed as the size of necrotic lesions, distance of fungal colonisation in the stem and the proportion of seedlings with dieback symptoms. Genotypic differences were found in both laboratory and greenhouse tests among various cacao genotypes, and the clones showed a wide range of disease reactions from highly resistant to very susceptible. The pathogenicity of F. decemcellulare and L. theobromae were similar in this study, which suggests that a breeding programme for controlling one of the pathogens can have benefit against the other. Direct significant correlations (r = 0.7) were obtained between visual dieback assessment scores and the percentage cross- sectional area of stem necrosis. Moreover, the response of inoculated stem segments corresponded to the reaction of intact plants despite the variation in the used methodology. Three cacao genotypes (CATIE 1000, T85/799 and MXC 67) were resistant or moderately resistant to F. decemcellulare and L. theobromae. These genotypes could be useful sources of resistance to both pathogens and other wilt causing pathogens in cacao. INTRODUCTION Ghana is the second largest producer of cacao (Theobroma cacao L.) globally after the Ivory Coast, and the crop generates some 30 to 40% of foreign exchange for the economy, providing employment to nearly a million people (Adu-Acheampong, 2009; Baah and Garforth, 2008). Species of Fusarium and Lasiodiplodia are important pathogens of cacao, causing substantial yield loss in most cacao producing countries in West Africa and inducing symptoms commonly referred to as dieback disease (Crowdy, 1947; Padi et al., 2001; Williams, 1953). The dieback disease, detected in Ghana in 1908 (Dungeon, 1910), is initiated by systemic growth of causal fungal pathogens via †Corresponding author. Email: r.aduacheampong@yahoo.co.uk; Present address: Cocoa Research Institute of Ghana, PO Box 8, Tafo-Akim, Ghana.