Morphological characterisation and in vitro management of thread blight pathogen(s) of cocoa (Theobroma cacao L.)

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Cocoa (Theobroma cacao L.) is the most important cash crop in Ghana, supporting the livelihood of a significant number of the Ghanaian populace, mostly farmers and rural dwellers. The increasing demand with concomitant increase in the price of cocoa on the world market is contributing significantly to raising the living standards of farmers. In recent years significant attention has been given to food security and the realization of the Millennium Development Goals (MDG). Decline in crop productivity as a result of increased rate of disease prevalence is one of the major constraints to agricultural productivity and realization of the MDGs in sub-Saharan Africa. A survey was conducted to assess the disease problem and to determine farmers’ knowledge about the disease. The disease was present on every cocoa farm surveyed and the incidence was between 50-55%. The disease is believed to be associated with some food crops. Six different isolates of thread blight were recovered. Mycelial growth rate, colony character and sporulation pattern of the fungal isolates, grown on seven different culture media viz., Potato Dextrose Agar (PDA), Malt Extract Agar (MEA), V-8 Juice Agar (V8), Oatmeal Agar (OMA), Plantain Agar (PA), Banana Agar (BA) and Green Cocoa Mucillage Agar (GCMA) were observed for seven days of incubation at 28±1°C. The cultural characteristics and sporulation pattern were greatly influenced by the type of medium used. Seventy different ages of cocoa seedlings and three alternative hosts were inoculated with each fungal isolate obtained from the farmers’ fields and symptoms expression observed. Symptoms appeared whitish on the stem and the leaf of the test plant. The pathogenicity test showed that white thread blight is responsible for causing most of the thread blight disease in cocoa. Eight different fungicides recommended on cocoa were used in vitro on the fungal isolates. Bioassay revealed that all the fungicides used inhibit mycelial growth of the test fungi to various degree, that increased in concentration of fungicides will increase percentage inhibition in the growth of the organism. The results showed Metalm 72WP and Fungikill were most effective on the thread blight pathogen.
Thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, in partial fulfilment of the requirement for the award of the degree, Master of Philosophy in Plant Pathology,