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of healthy cacao pod (top) and a pod ruined by the witches' broom fungus.
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Cocoa's Two Most Devastating Pathogens Are Close
Kin By Erin
Peabody October 12, 2005
Among certain related fungi, a love for chocolate apparently runs in
the family, according to scientists with the Agricultural Research Service. The
scientists have found that the two most serious fungi infesting the
worlds cocoa plants are actually sisters, taxonomically
Using DNA analysis,
Aime, a molecular biologist at ARS
Botany and Mycology Laboratory in Beltsville, Md., found that the two
fungal pathogens causing witches broom and frosty pod rot are close
relatives that really should be classified in the same genus. Wilbert
Phillips-Mora of CATIE in Costa Rica
assisted Aime in the research.
Witches broom, caused by the fungus currently known as
Crinipellis perniciosa, and frosty pod rot, brought on by
Moniliophthora roreri, are the two greatest threats to the worlds
chocolate supply. Despite widespread fungicide applications, these diseases
continue to invade new parts of South and Central America, discouraging the
mostly small farmers who raise cacao trees.
Despite the pathogens notoriety, relatively little is known
about the two fungi or their life cycles, making the challenge of their control
even more difficult.
The witches broom fungus lives inside the cacao plant, causing
it to haphazardly send out deformed, broomlike shoots. Frosty pod looks more
like a cottony mold covering the cacao trees seed-bearing pods.
A colleague of Aimes, Harry Evans of
CABI Bioscience in the United Kingdom, was
one of the first mycologists to speculate that the two fungi might be closely
related. But at the time, he lacked the molecular tools needed to conclusively
determine the organisms connectedness.
For the current research, Aime sequenced several genes from the two
cocoa pathogens. Almost immediately, she could see that witches broom and
frosty pod were both members of the order Agaricales, the mushroom-forming
fungi. Further RNA analysis revealed even more detailed evidence pointing to a
close, sister-species relationship between the two.
Aime has proposed how to reclassify the two fungi, which, it now
appears, share the same ancestor.
learn more, read the current issue of Agricultural Research
ARS is the U.S. Department of
Agricultures chief scientific research agency.