Classical biological control of cocoa diseases
United States Department of Agriculture (USDA)
Centro Agronomico Tropical de Investigacion y Ensenanza (CATIE)
Problem/Issue. Cacao (Theobroma cacao) has been proposed as an ecologically and environmentally ideal alternative crop for the replacement of narcotic cultivation, principally coca (Erythroxylum coca) in South America. The main biological constraint to the sustainable cultivation of cacao in Latin America is fungal disease, in particular frosty pod rot (Moniliophthora roreri) and witches’ broom (Crinipellis perniciosa). M. roreri infects pods and results in complete destruction of the beans. C. perniciosa infects meristematic tissue producing vegetative or cushion brooms, chirimoya-like pods and, if it infects the pods, also leads to the destruction of the beans.
Measures to Date. These pathogens are both still in an invasive phase and the use of conventional chemical (copper fungicides) and cultural control techniques (phytosanitary pruning) have failed to halt the progress of these diseases.
Project Activities. At our Ascot site, two distinct approaches are being investigated: the use of mycoparasites/antagonists, and the use of endophytes. Mycoparasites/antagonists can be exploited to reduce inoculum pressure of M.roreri by direct antagonism and/or parasitism (competitive exclusion and induced resistance may also be important). Endophytes (mutualistic symbionts) have the potential to protect their host plants through multiple mechanisms: competitive exclusion; induced resistance; antagonism or mycoparasitism. Both approaches are being pursued using the principles of classical biological control. Classical biological control has been successfully used to control invasive weeds and alien pests. This is thought to be the first instance of classical biological control being deliberately targeted at plant pathogens (the recent release of Trichoderma stromaticum to control C. perniciosa in Bahia, Brazil is an “accidental” use of classical biocontrol tactics). Classical biological control involves returning to the centre of origin of the target pest organism to find natural enemies, in this case mycoparasites/antagonists, which have co-evolved with it. For endophytes, collections are made in the centre of origin of cacao (the Amazon basin) and the strategy is based on the premiss that, in natural forest ecosystems, specific endophytes have co-evolved with cacao and its Theobroma and Herrania relatives but that these are absent or have been filtered out in exotic agroecosystems.
Achievements So Far
Surveys for mycoparasites/antagonists have been undertaken in the suspected centres of origin of the pathogens, Western Ecuador (M.roreri) and Amazonia (C. perniciosa). Isolation of the mycoparasites were made directly from infected pods or brooms. Endophytes were collected in natural forest ecosystems in Ecuador, Peru, Brazil and Mexico on a range of wild Theobroma and Herrania spp, including T. cacao, as well as in exotic cacao agroecosystems in Brazil (Bahia), Costa Rica, India and Ghana. It appears that endophytes from natural forest ecosystems have a range of unusual fungi such as Mortierella, Arxiella, Stagonospora, Glonium, Acremonium sp. Isolates from exotic agroecosystems appear to be dominated by Penicillium, Coniothyrium, Gliocladium and Botryodiplodia spp. To date over 2,000 endophyte isolates and 100 candidate mycoparasites have been collected. Endophytes are initially screened for their ability to colonise cacao, using a seedling assay. Initial studies demonstrated the ability of the endophytes to colonise cacao, with their successful re-isolation from symptomless meristems. Collaborative studies undertaken with Almirante Cacau in Brazil have shown that the introduced endophytes are able to reduce the incidence of witches' broom disease in seedlings. Small-scale field studies have now been initiated in Ecuador to assess the ability of endophytes to colonise and persist in the growing cacao pod.
Initial investigation of the candidate mycoparasites identified a number of potentially useful mycoparasites. Empirical field studies undertaken in Costa Rica to assess selected mycoparaistites demonstrated their ability to colonise and persist on the cacao pod.
In the near future the field screening of the endophytes, in Ecuador, will be assessed, with new agents being screened and those successful in the first studies being assessed for the ability to reduce frosty pod rot incidence. In Costa Rica the mycoparasites will be assessed for their ability to reduce incidence of frosty pod rot in a field trial.
Dr. Harry C. Evans
Start date: October 1997
End date: March 2005