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Chocolate Under Siege: Plant Pathogens Threaten World Cacao Supply

Marie A. C. Langham
South Dakota State University

What is the world's favorite sweet? Would it be candy, cake, pie, ice cream, or drink? There would be thousands of competitors for this title, but many would have one thing in common. They would be flavored with chocolate. No matter how it is prepared, the world loves chocolate. Our passion for chocolate began with the Aztec and Mayas and has grown until today more than a million tons of cocoa beans are harvested each year for chocolate production. Chocolate has become a part of our culture and is associated with romance, comfort, and tradition. Could you imagine celebrating holidays or mending a broken heart without chocolate?

Unfortunately for chocolate lovers, some plant pathogens also have a "taste for chocolate" or at least for the cacao trees that produce chocolate. Cacao trees evolved as one of the understory trees in many of the world's rain forests. Commercial cacao trees are grown in plantations in tropical regions of the world, such as South America, Central America, Africa, and Asia. When its pods ripen, farmers collect the seeds from the cacao pod and then ferment and roast them. It is these "cocoa beans" that are ground to produce chocolate, cocoa butter, and cocoa. Three widespread and two localized diseases are threatening production of the world's chocolate supply. These five diseases cause an estimated annual reduction in world cocoa bean production of 810,000 metric tons worth $761,000,000 (US).

Black pod, caused by several species of Phytophthora, is the most widespread of these diseases and is found in all cacao growing regions of the world. However, it is most severe in West and Central Africa. Black pod may affect any portion of the tree, but infection of the pods causes the most devastating losses. On the pods, it produces a spreading dark brown lesion that eventually covers the entire pod. Infection in the pod moves from the outer husk to the cocoa beans causing discoloration and shriveling. Diseased pods eventually become black and mummified. Black pod causes an annual loss estimated at 450,000 metric tons of cocoa.

Witches' broom, caused by Crinipellis perniciosa, is found primarily in South America although, it has also been reported in Panama and some Caribbean islands. This pathogen is a Basidiomycete fungus, and it produces pink, mushroom-like growths on infected trees. The fungus changes the physiological and hormonal relationships in the cacao tree causing it to grow clusters of succulent branches called brooms from flower clusters or other parts of the plant. It can also infect the cacao pods. Witches' broom reduced the Brazilian production of cocoa beans by 75% in less than ten years.

The third major disease of cacao is frosty pod rot, caused by Moniliophthora roreri. Conidia (asexual spores) of this fungus typically infect the pods within the first 90 days of their development. The pods soon develop discolored and swollen areas that are quickly covered with a dense layer of light colored spores. It is the layer of spores that gives the pod its "frosty" appearance. Frosty pod rot has been found in Ecuador, Peru, Colombia, Nicaragua, Costa Rica, and Panama. Future pathogen spread may also threaten cacao production in Brazil, one of the largest production areas in South America.

The final two diseases threatening world cacao production are swollen shoot caused by Cacao swollen shoot virus (CSSV) and vascular-streak dieback caused by the fungus, Oncobasidium theobromae. CSSV affects cacao production in West Africa where it causes an annual loss in cocoa beans of 50,000 metric tons. Vascular-streak dieback is limited to Asia where it annually causes a loss of 30,000 metric tons of cocoa beans.

How can we combat these diseases that threaten our chocolate? Plant pathologists are fighting for the cacao tree with many techniques and methods. Progress is being made in providing practical fungicide treatments and biological control agents. Plant pathologists are also developing management strategies such as frequent harvesting to reduce yield loss with black pod. Genetic and induced resistances are being tested for their use in the "chocolate" defense strategy. Finally, other pathologists are trying to genetically modify the cacao tree to resist plant pathogens. So, the future of our chocolate and the cacao becomes more secure with each new discovery. This is great news for the whole world and not just for chocolate lovers, because in addition to producing chocolate, the cacao tree is being promoted as an essential component in preserving the world's rainforests. Thus, the rewards for saving the cacao tree may be "sweeter" than simply saving our chocolate supply.

For more information about cacao trees and chocolate, try the following websites:

• From the World Cocoa Foundation, The Cocoa Tree at http://www.cocoatree.org/ provides an educational overview of cacao trees, life on a cacao farm, how cocoa beans are made into chocolate bars, and the sustainability of cacao farming.
  
  
• Chocolate: The Exhibition sponsored by the Chicago Field Museum (http://www.fieldmuseum.org/chocolate/index.html) has information and interactive pages on chocolate. It also includes educational resources for educators.

For further information about the diseases threatening cacao and the production of chocolate, more details can be found at these websites:

• Available in the APSnet Feature Story archive, The Impact of Plant Diseases on World Chocolate Production provides a detailed discussion of the disease of cacao and their impact on world chocolate production and is the source for the yield loss estimates used in this article.
  
  
• If you missed Dr. Randy Ploetz speaking on cacao diseases and their impact on the Discovery Channel’s News Capsule, you can still catch it at Daily Planet for February 13, 2004.
  
• Fighting a Fungal Siege on Cacao Farms from the USDA Agricultural Research Services is an overview of ARS research programs on cacao diseases (http://www.ars.usda.gov/is/AR/archive/nov99/cacao1199.htm).

Views: The pink basidiocarps shown on the cacao branch and pod are reproductive structures formed by the fungus, Crinipellis perniciosa. This fungus causes witches' broom in cacao trees.