RESISTANCE TO DIEBACK DISEASE CAUSED BY FUSARIUM AND LASIODIPLODIA SPECIES IN CACAO (THEOBROMA CACAO L.) GENOTYPES

Abstract

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 crosssectional 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.

Optimal pheromone trap density for mass trapping cacao mirids

“…All mirid active stages feed on cacao stems and pods causing little direct damage. However, their phytotoxic saliva results in sunken lesions that become infected secondarily by parasitic fungi that cause shoot die back (Johnson, 1962;Adu-Acheampong et al, 2012). Damage varies from mild 'blast' where leaves on fan branches die but remain hanging, through 'staghead' where the whole canopy of individual trees is defoliated and shoots die back, to the most severe 'pockets' where up to 100 trees become stagheaded with corresponding breaks in the cacao canopy (Williams, 1954;Johnson, 1962).…”

Untitled

“…Several factors such as the length of the lesion in cambium, wilting and yellowing of the leaves can be used to determine the resistance of the cocoa genotypes to dieback. Adu Acheampong et al (2012) showed the existence of a direct correlation between visible symptoms of dieback and the lengths of necrosis in the cambium. Indeed, cocoa varieties with a large lesion length are more susceptible to dieback and those with a small length of lesion tolerant.…”

“…Chemical and biological control methods were usually practiced in order to prevent the plant diseases and to protect the crop plants against pathogens. After more than a century experience of controlling dieback disease, the ideal control regime is still to be found (Adu-Acheampong et al, 2012). Implementation of integrated disease management (IDM) programs which combine cultural, chemical, and biological approaches are highly recommended to control crop diseases, reduce cost, and improve production efficiency.…”

Visual cues from different trap colours affect catches of Sahlbergella singularis (Hemiptera: Miridae) in sex pheromone traps in Cameroon cocoa plantations

“…and Lasiodiplodia spp. (Adu-Acheampong et al, 2012Voula et al, 2018) that can lead to cocoa dieback (Anikwe et al, 2015). There is also mounting evidence that yield losses caused by mirid feeding are principally due to abortion of young fruits (Yede et al, 2012).…”