Endophytic Bacillus Spp. of Theobroma cacao: Ecology and potential for biological control of cacao diseases

Abstract

In South America, there are three key diseases that affect the yield of Theobroma cacao: black pod, caused by Phytophthora spp.; frosty pod, caused by Moniliophthora roreri; and witches' broom, caused by Moniliophthora perniciosa. Although chemical control options exist, farmers typically only use cultural disease management such as phytosanitary pruning. Agrochemical use can be problematic in the developing countries where cacao is grown due to large risks to human health and the environment. As a result, there has been increased interest in the use of biological control for management of cacao diseases due to problems associated with pesticide use as well as consumer desire for organic chocolate. Fungal species have been the main focus for research on biological control of cacao diseases, while cacao-associated bacteria have been nearly ignored. The research in this dissertation focuses on obtaining and screening endospore-forming cacao bacterial endophytes for their ability to suppress diseases. In addition to research on suppression of witches' broom, four bacterial isolates were tested for their ability to suppress cacao pod disease and cherelle wilt in two cacao genotypes. None of the four tested isolates reduced diseases on cacao pods, but application of B. pumilus ET increased the overall number of healthy pods in the first two months of the four month experiment. These successful field results indicate the positive potential for the use of native endophytic bacteria to manage cacao diseases. In conclusion, the results reported here indicate that B. pumilus ET would likely make an excellent biological control agent, due to its multiple modes of action and ability to be combined with endophytic Trichoderma spp.