Theobroma Cacao – The Need for Hybridisation

by Melissa Cotterell

World demand for chocolate is still growing, with new markets opening up in China and the Far East and demand expected to increase by around 25% within the decade. The cacao industry is unable to cope as it stands at present, with outmoded farming practices and lack of disease and pest-resistant hybrids taking their toll. While for many years small producers have struggled to eke out a living from cacao farming, the industry is just beginning to adapt to the changing face of cocoa farming by the development of sustainable practices.

The Origin of Cacao

Cacao originated in the Americas and spread throughout the globe to Africa and South East Asia. The tree grows only in the tropical and sub-tropical belts of the Equator, although there are regional differences in the cocoa produced due to climate and soil. Venezuelan cacao is perhaps considered the best in the industry, although the majority of the world’s cacao crop is produced in Ghana and the Ivory Coast. Colombia is pushing its cacao industry to become a major export. Cacao has been identified as a viable high cash flow crop to replace illicit drug cultivations in Colombia and elsewhere in South America.

Varieties of Tree

There are four main varieties of cacao: Criollo, Forastero, Trinitario, and Nacional. The Criollo is typically low cropping, but of excellent quality and the beans from this tree are often used in a blend for the production of chocolate. The Forastero is the most common variety in the industry, producing around 80% of the world’s annual output. This variety is favoured due to its high yield and rapid growth, as well as resistance to disease. The Trinitario is a hybrid of the Criollo and Forastero, predominantly cultivated in Central and South America, as well as in Asia. The Nacional is vulnerable to disease and more difficult to cultivate, but has a much sought-after aroma.

Pests and Disease

The cacao industry is subject to a number of different pests and diseases, which has lead to losses of up to 30-40% of global production each year. Heavy research has been devoted to minimizing this damage by extensive study into zoology and identifying animals such as the Capuchin monkey, who like to eat the soft pulp surrounding the cacao seeds from which we produce chocolate. The seeds themselves are poisonous to animals due to their theobromine content and are rejected in favor of the sweet flesh. Theobromine is an alkaloid from the same family as caffeine, contained in the cocoa butter from which chocolate is made. For this reason, commercial crops do not suffer damage from animal foraging in the same way that other crops such as fruit crops might.

It is the damage from the infestation of different varieties of rot and fungi, as well as insects such as the Cocoa Moth, which are responsible for the problems in cacao plantations worldwide. Cocoa mirids bore into cocoa stems, pods and branches, killing the cells they enter, which then produce necrotic lesions. Mirid infestations on shoots frequently cause destruction on the tree as the terminal shoots and leaves die. They prefer trees which are growing in sunlight, although once they have colonized they will spread into trees which are shaded. Insecticide use is common, although interplanting with indigenous plant species, which the pest finds suitable as a host, combined with reduced use of insecticide to allow other insects to prey on the mirids allows for biological control. The Cocoa Pod Borer (or Cocoa Moth) caused widespread losses in the industry during 1890’s and 1900’s. This pest now affects virtually all cocoa producing areas in Indonesia, which has led to decreased production in Malaysia. Although pesticides are effective in controlling this pest at sustainable levels, the high cost of spraying makes this type of control nonviable.

 Witches Broom disease is a fungal infection, which spread throughout the cocoa-producing regions of South America, Panama and the Caribbean. The losses sustained by the industry have perhaps been heaviest in Brazil, where for a decade the Bahia region suffered a 70% loss of production. Although some Trinidad hybrids were developed in the 1950’s which showed resistance to the disease, more aggressive strains of the pathogen from other regions has made these hybrids ineffective. Frosty pod rot is prevalent throughout the Latin American cacao region, causing significant losses and leading to the abandonment of farms. All cacao species appear vulnerable to this disease.  

Black Pod Rot (Phytopthera) has three fungal species of the same genus: P. palmivora, P. megakarya, and P. capsici. P. palmivora and is responsible for global crop loss of around 20-30%; P. megakarya is the most aggressive of these three pathogens, most prevalent in Central and West Africa; while P. capsici is widespread throughout both Central and South America, where it causes significant losses. Vascular-streak die back decimated mature plantations in Papua New Guinea during the 1960’s and has since spread through South East Asia, where it continues to cause major losses.

Cacao Genome Sequenced

At the end of the first decade of this century, an international team led by Claire Lanaud of CIRAD sequenced the DNA of the Criollo variety of Theobroma cacao, identifying a variety of gene families which it is hoped will contribute to the improvement of the cacao tree and fruit – whether by improving growth and crop yield or by enhancing resistance to disease and pests.

Development and Distribution of Resistant Varieties

Through projects such as Cocoa Productivity and Quality Improvement: A Participatory Approach, a number of high-yielding and resistant varieties of cacao have been released to farmers, while research continues into new and improved cocoa planting materials for future release. This global project has been successfully implemented in 13 different countries.

Approximately 2,000 farms across ten different countries were surveyed, with around 2,000 trees identified by the farmers as resistant being of interest due to their high resistance to disease and infestation, or for their high crop yield. Out of these, about 1,500 farm selections had in-situ observation plots established, or had on-farm trial plots set up across 8 countries. The genetic diversity of several thousand farm selections in Africa were analyzed, showing great genetic variation in farm populations, which are of mainly hybrid origin (with important contributions of the Amelonado, Trinitario and Upper Amazon parental genomes).

The project established approximately 240 on-farm selection plots, with varieties selected by breeders being compared with farm selections (clones or seedling progenies). The number of plots has since declined through farmer neglect or drought to around 120, which are still being observed. A number of new varieties have already been selected or confirmed for distribution to farmers, while others have been selected for ongoing trials in other cacao growing regions.  New variety trials have been implemented in: Brazil; Ivory Coast; Ecuador; Colombia, Malaysia; and Papua New Guinea.