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Breeding a better cocoa plant for chocolate lovers.

Smooth, rich. creamy. delicious. Melts in your mouth. Just a little chunk of it can give a quick burst of energy and relieve tensions trod frustrations. Chocolate...according to a recent Gztllup survey, it's Americans' favorite flavor.

But where does cocoa-the main ingredient in such deliciously decadent sweets as chocolate cream pie, mousse. and filled and coated candies-come from? Not from anywhere in the United States, but from Theobroma cacao trees that grow in Central and Sotith America. Africa, the West Indies, Oceania, and Asia.

Raymond J. Schnell, ARS horticulturist at the U.S. Horticultural Research Laboratory in Miami. helps keep these trees healthy and productive. His research is an important link in the chain of events that brings raw products to U.S. chocolate manufacturers.

Through technology transfer, Schnell's research could help improve production and quality of cocoa worldwide. "Over the past 30 years, we've been trying to understand the genetics of T. cacao. But attempts to breed more productive and diseaseresistant cacao haven't been very successful," he says.

"Witches' broom is one of the most ravaging diseases to hh cacao plants," he says. "We're working now to produce resistant trees."

In some northern areas of South America, losses from witches' broom can be almost 100 percent. This is the case in the region of Quevedo, Ecuador.

At the ARS Miami lab, Schnell tends about 200 cacao trees that he uses as parents of seedlings that he screens for resistance to witches' broom.

Schnell has traveled extensively in Central and South America. where cacao evolved, to bring back resisrant plants that might help against this disease.

"I visit countries where there are cacao trees that seem to escape this disease and bring back germplasm," Schnell explains. "We then crosspollinate with trees growing here on the research station."

After a cross is made, it takes about 8 months to produce a seedling. From seedling to first harvest can take 5 years. with larger harvests requiring 5 to 8. So cacao breeding takes time. Given about 20 years, mature trees eventually grow to heights of up to 35 feet.

Schnell is looking for genetic markers that are linked to diseaseresistance genes.

"In this analysis, we use protein and DNA from the plant in somewhat the same way that DNA from humans is used to detect predisposition to certain diseases."' he says.

Catherine Ronning, a graduate student working with Schnell on cacao research, says, "If we find a particular enzyme pattern that appears to be correlated with disease resistance, then we can use this pattern as a genetic marker in screening for resistance to witches' broom. These markers will greatly reduce the amount of time required for disease screening."

"We have some very promising results from apparently resistant plants we brought back from a severely infested area of Ecuador in 1989," SchneH says. "We expect to release witches' broom-resistant stock to breeders within the next 2 or 3 years."

Witches' broom is caused by a fungus, Crinipellis perniciosa, according to Schnell's collaborator L.H. Purdy, who is with the University of Florida's Department of Plant Pathology.

"If a new branch of a cacao tree looks distorted with shoots overgrown, then you can be sure the fungus is at work," Purdy says. "The new growth configuration from an infected tree looks like a bloated hand with swollen, distended fingers."

It's this strange-looking, fanlike structure resembling a broom that gives the disease its name. The broom, which contains the fungus, remains on the tree for a month or two, then tums brown, dies, and either stays where it developed or falls to the ground or lodges in the cacao tree.

"Wind carries the spores, and water activates the fungus," Purdy says. "Therefore, rainfall hitting these brooms (either on the tree or ground) stimulates production of small mushrooms that produce spores, which are the only infective part of the fungus. These spores reinfect other places on the tree or can float on the wind to infect other trees."

When they land on a wet tree, the spores germinate in about 2 hours and can penetrate plant tissue, causing infection in about 8 hours.

Cutting out the brooms from the trees would help prevent reinfection. So breeding trees to reach a more manageable height-perhaps 12 to 15 feet-would be a significant help.

Cacao produces flowers differently from most other trees. Instead of forming on branches like an apple or peach, buds grow directly on the tree trunk. Tiny cushions about the size of a quarter appear on the trunk. It's from these cushions that flowers and pods come. The pods remain on the plant for about 6 months; for the first 3, they are highly susceptible to witches' broom. If fungal infection occurs then, the cacao beans in the pods become unmarketable.

A chemical control is available, but the cost makes it impractical. Most cacao producers are harvesters, not farmers. They don't cultivate the trees, just harvest the beans. Income from the beans may be their only income.

Also, the great height of the trees and hilly terrain in some countries make it difficult to spray, even if the harvesters could afford it. Then too, Purdy says, the chemical is a protectant. If the tree is already infected, spraying will neither stop the fungus nor cure the tree.

Although new systemic fungicides are available, the U.S. Environmental Protection Agency has not approved their use on cacao. Both Purdy and Schnell agree that finding resistant plants is the best answer. In addition to resisting witches' broom, they're also looking for a T. cacao variety that will yield well.

The American Cocoa Research Institute of the Chocolate Manufacturers Association and the U.S, Agency for International Development are collaborators on Schnell's research.By Doris Stanley, ARS.

Raymond J. Schnell is at the USDAARS Subtropical Horticulture Research Laboratoty, 13601 OM Cutlet' Road, Miami, FL 33158. Phone (305) 254-3632.
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Title Annotation:includes related article on cocoa facts
Author:Stanley, Doris
Publication:Agricultural Research
Date:May 1, 1992
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