Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/11/02
Contacts:
Paula Flynn, Plant Pathology, (515) 294-0581, pflynn@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning February 15, 2002
Hot Chocolate, Hot Commodity?
Paula Flynn
Extension Plant Pathology
Chocolate is a luxury food we often take for granted. You can grab a handful of chocolate bars as you pass through the checkout lane in just about any grocery or department store. At this time of year there are countless red, heart-shaped boxes full of every shape and kind of chocolate imaginable.
Imagine a shortage of chocolate, or maybe paying a much higher price for a candy bar. The cacao tree, which produces the beans used for making chocolate, has been increasingly threatened by disease problems. Although there's no need to dash to the store and stock up on your favorite treat, it's a reminder that plant diseases can affect many aspects of our daily life.
The cacao tree is native to South America and Central America. The scientific name for the tree, Theobroma, means "food of the gods." Cacao trees grow in the rainforest under the protection of taller trees. Pods, shaped like small rugby balls, grow on the trunk of these trees. Inside the pods are 20 to 60 beans or seeds. These cocoa beans are collected, dried and eventually processed in a variety of products that range from candy to cocoa butter lotion.
The pods take five to six months to mature, giving natural enemies like fungi plenty of time to attack. The humid environment of the rainforest also provide a nice environment for disease-causing fungi. Frequent rains provide a convenient means for fungal spores to move from place to place.
Disease problems on cacao have become more common for several reasons. In a tropical rainforest, cacao trees are scattered and diseases don't spread as easily as in a cacao plantation where the trees are crowded together. Without the protective canopy of taller rainforest trees, the cacao may show symptoms of stress. The insects that pollinate the cacao flower prefer the high humidity of the natural habitat and don't fare well in more open areas. Diseases also have been introduced into new areas with the movement of trees between countries.
The common names of the fungal diseases that feast on the cacao tree are quite descriptive - frosty pod rot, black pod and witches' broom. The fungus that causes frosty pod rot attacks the pods and later covers them with light colored fungal structures, giving them a misshapen, frosty appearance. The black pod fungus works on the cocoa beans inside the pod, causing them to shrivel. The entire pod eventually turns black and shrivels. The end result is referred to as a mummified pod. The witches broom fungus causes pods to deform, but also causes the tree to produce many small branches called brooms. This unnecessary formation of branches takes energy away from the normal growth and development of the tree.
What can be done to protect cacao trees from being wiped out by disease? As with many plant health problems, combining current research with management tools that are already available is the best approach. This is often referred to as integrated crop management or integrated pest management. Good sanitation practices are a key component of an integrated management approach. Diseased trees, branches or pods can be promptly removed to prevent spread of the fungus. Fungicides are too expensive for most small growers, and frequent rains in the tropics often wash them away before they can be effective.
Researchers are looking for other living organisms to combat cacao diseases. One effort is to find natural enemies of the harmful pathogens. These natural enemies, such as "good" fungi, also thrive in the rainforest. Other naturally occurring fungi work to protect the cacao tree by excluding disease organisms from infecting a tree or by stimulating the tree to become less vulnerable to pathogen attack. Once they prove their worth, these helpful fungi could be sprayed onto cacao trees to slow or stop disease. Finally, work is being done to identify, develop and enhance genetic resistance in wild and cultivated populations of cacao.
While this tree keeps the world supplied with chocolate, it serves other important uses. Harvesting its beans provides income for small farmers and their families. It also supports animals, insects and plants as an integral part of the rainforest ecosystem .
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