A Brief Survey of the Spread of Cacao Plants and Pathogens

The cacao plants were domesticated for their pulp and seeds in Mesoamerica by the Olmecs (1200-400 BCE) and the Mayans (500 BCE -900 CE).  This cutigen, popularly known as criollo (local), is but one variant of the species Theobroma cacao.  At its ancestral home in the upper Amazon basin in Peru, Ecuador, Columbia and Brazil, multitudes of wild cacao genes coevolved with pathogens.  As a cultigen with limited genetic strains and prolonged inbreeding, criollo was susceptible to a variety of disease and pests.  Today, criollo has all but disappeared from the pre-Hispanic Mexico. (Presilla p.23)

The principal strategy for meeting ever-increasing demand for cocoa products in the face of withering cacao disease has been to continuously look for new regions to plant the trees.  As cacao plants spread around the world within 20 degrees latitude of the equator, they were threatened by indigenous disease and pests along with the old pathogens that eventually caught up with them. The dispersal of cacao cultivation and the shift in production centers often followed catastrophic failures of cacao farms in one region or another.  Some of the large-scale crop failures that occurred in the 17th century Venezuela (mid-1630s), Cuba (1660s-1670s), Hispaniola (1660s 1670s), Jamaica (1670s) and Martinique (1671) were probably due to blight of some sort (or “blast” as it was called). (Zhang and Motilal p.9-10; Coe and Coe p.194)  The cacao plants of the West Indian islands were hit particularly hard in the 1720s.  In what turned out to be of historic importance, Capuchins introduced a cacao strain from the Middle Orinoco region on the mainland to Trinidad some thirty years after the catastrophic failure of the cacao farms on the island in 1725 and 1727.  This strain which they called forastero (from the forest or foreign) interbred with the surviving criollo strain on the island and morphed into a new breed, trinitario, which retained the flavor of criollo and the hardiness and high yielding traits of forastero. (Coe & Coe p.196)  This new breed would later drive out the prized criollo cacao of Venezuela after the outbreaks of disease there in the early nineteenth century. (Presilla p.93)

The humid habitat of cacao plants is also a perfect breeding ground for some of the most serious pathogens that afflict them.  The cocoa yields at a stricken farm may be reduced by 30-90 percent by one such species, Phytophtora, which causes black pod rot, taking an annual toll of 450,000 tons or over 10 percent of the worldwide production of cocoa.  One particularly virulent species, P. megakarya, is seriously threatening West African region, today’s biggest cacao growing area.  Crinipellis perniciosa, the fungal species which causes witches’ broom, was first reported in 1895 in Suriname and, today, it is responsible for the annual loss of 250,000 tons.  The 20th century witnessed many important cacao producing regions succumbing to diseases.  Ecuador’s cacao farms which suffered throughout the century was hard hit in the 1920s by witches’ broom and frosty pod infestations which cut the cocoa production by 60-70 percent.  The El Nino of 1982 to 1983 and 1997 to 1998 nearly finished off what was left of the cacao trees there.  In Brazil, witches’ broom and black pod rot made their appearance in the 1950s.  The black pod rot caused a great havoc  in the 1970s for Bahian cacao growers who produce 85 percent of Brazil’s cocoa.  In the 1990s, Brazil’s annual production fell to 100,000 tons from 400,000 and the country, once the largest cocoa producer, 24 percent of the world total in 1983, became a net importer.  On the Portuguese colony of São Tomé and Principe where cacao was first introduced in 1822, the swollen shoot disease had a far greater effect on its economy than the anti-slavery inspired 1909 boycott by Cadbury, Fry, Rowntree and Stollwerck.  It’s cocoa export fell 56 percent in 1918 from the previous year, from 27,127 to 12,074 tons, and the remaining cacao buyers soon turned to Gold Coast (today’s Ghana) and Nigeria for supplies.  Soon this same disease would affect the cacao farms in Ghana as well, leading to the loss of 185.5 million trees from the 1950s to 1980s.  The swollen shoot, caused by Moniliophthora roreri, is still cutting into the cocoa harvest by 50,000 tons annually, mostly in Africa. (Bright p.17-18; Zhang and Motilal p.5, 12, Table 1.1; Presilla p.49, 123; Higgs p.148, 160; Bowers and Bailey)

Various strategies have been deployed to forestall the ravages of cacao plant diseases.  Chief among them is the effort to develop cacao strains that are resistant to infections.  An important contribution was made by F. J. Pound and others of the Imperial College of Tropical Agriculture in Trinidad who collected the wild cacao strains of the western tributaries of the Amazon since the late 1930s.  Promising strains were cloned  and crossbred for disease resistance, yields or other desirable traits. (Presilla p.48)  Much of today’s cacao production around the world relies on the seeds and the clones developed from the Pound collection.  Yet this collection represents but a small sample of what’s growing in the wild.  In the face of the rapid Amazonian deforestation which poses the greatest threat to the cacao genetic diversity, new expeditions are under way to fill in the gaps in the genetic jigsaw puzzle in the exiting collections ex situ.  It has been a time-consuming process as it involves field testing of the plants from seedlings to mature trees for suitability at various locales under different environments.  Genomic studies are expected to offer lab-based testing and more efficient development for disease resistant cacao breeds. (Moyer)

The spores of the fungi that attack the plants may stay dormant in the soil or debris for many years and they are easily moved around by wind or by hitching themselves to shoes.  Strict quarantines are instituted in the biggest cacao producing regions like Cote d’Ivoire and Ghana.  For a plant species that resists mechanization in care and harvesting, the most cost effective disease and pest suppression regime seems to be the frequent removal of affected cacao pods, branches and leaves. (Zhang and Motilal p.6)  The use of chemical fungicides or pesticides may cause long-term adverse effects on the health of the environment, the farmers and the consumers. Additionally, their cost may be prohibitive to small holder subsistence farmers who dominate the cacao production.  As an alternative to the use of chemicals, some growers in Bali have found biological defense useful.  There, hoards of black ants are let loose to gorge on cocoa pod borers or cocoa moth, Conopomorpha cramerella. (Presilla p.121)  The large-scale cacao plantations, like the proverbial eggs in one basket, present particularly vulnerable targets to the fungal spores that spread rapidly with devastating consequences.  The small scale, labor intensive farming in a biologically diversified habitat as practiced in the pre-Columbian Mesoamerica seems uniquely suited to the healthy survival of the species and the industry that depends on it.

Black pod disease of cacao

Black pod rot caused by Phytophthora capsici (Bowers and Bailey)

A dry broom formed from infection of an apical vegetative bud

Witches’ broom formed by Crinipellis perniciosa. (Bowers and Bailey)

Infected cacao pod showing surface lesion and extensive seed rotting

The effect of C. perniciosa on the cacao pod and seeds. (Bowers and Bailey)

Frosty pod rot of cacao

Frosty pod rot of cacao caused by Moniliophthora roreri. (Bowers and Bailey)

FullSizeRender-3

Ants feasting on cacao pod borers, Conopomorpha cramerella. (Presilla p.121)

Bibliography

Bowers, John H. and Bryan A. Bailey. “The Impact of Plant Diseases on World Chocolate Production,” Plant Health Progress. June 14, 2001. doi:10.1094/PHP-2001-0709-01-RV.

Bright, Chris. “Chocolate Could Bring the Forest Back,” World Watch. v.14. #6. 17-28, Worldwatch Institute, November/December 2001.

Coe, Sophie D. and Michael D. Coe. The True History of Chocolate (3rd edition). London: Thames and Hudson Ltd., 2013.

Higgs, Catherine. Chocolate Islands: Cocoa, Slavery and Colonial Africa. Athens, Ohio: Ohio University Press, 2012.

Moyer, Michael. “Death and Chocolate,” Scientific American 303, 21. October 2010. doi:10.1038/scientificamerican1010-21b.

Presilla, Maricel E. The New Taste of Chocolate: A Cultural and Natural History of Cacao with Recipes. (revised) Berkeley: Ten Speed Press, 2009.

Zhang, Dapeng and Lambert Motilal. “Origin, Dispersal, and Current Global Distribution of Cacao Genetic Diversity,” in Cacao Diseases: A History of Old Enemies and New Encounters, eds. Bailey, Bryan A. and Lyndel W. Meinhardt, 3-32. Switzerland: Springer International Publishing, 2016. doi: 10.1007/978-3-319-24789-2.

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