Submitted to: Mycological Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 10, 2009
Publication Date: December 1, 2009
Citation: Bailey, B.A., Strem, M.D., Wood, D.F. 2009. Trichoderma species form endophytic associations within Theobroma cacao trichomes. Mycological Research. 113:1365-1376. Interpretive Summary: Yields from Theobroma cacao (cacao), the source of chocolate, are severely limited due to devastating diseases. The beneficial fungus Trichoderma can protect plants from diseases. Although Trichoderma is normally applied to the soil, we are applying Trichoderma to the leaves, stems, flowers, and fruit of cacao with the goal of controlling major cacao diseases. It is unknown how Trichoderma grows on and in these plant parts, but this information is key to understanding how Trichodermas protect plants from disease. In this manuscript we discovered that Trichoderma can grow on and in the trichomes of cacao allowing it the opportunity to protect cacao from disease. Trichomes are the small hairs that cover the surface of many plants. Using this knowledge it may be possible for scientists to optimize control measures for cacao diseases by insuring that Trichoderma, once applied, becomes established at high populations on and in cacao tissues. This would benefit cacao growers and chocolate producers around the world by minimizing crop losses due to disease and helping stabilize cacao supplies.
Technical Abstract: Trichoderma species used in biological control of plant disease are usually considered soil organisms that colonize plant roots, sometimes forming a symbiotic relationship. Recent studies demonstrate that Trichoderma species are also capable of colonizing the above ground tissues of Theobroma cacao (cacao) in what has been characterized as an endophytic relationship. Trichoderma species can be re-isolated from surface sterilized cacao stem tissue, including the bark and xylem, the apical meristem, and to a lesser degree from leaves. SEM analysis of cacao stems colonized by isolates of four Trichoderma species (T. ovalisporum-DIS 70a, T. hamatum-DIS 219b, T. koningiopsis-DIS 172ai, or T. harzianum-DIS 219f) showed a preference for surface colonization of glandular trichomes versus non-glandular trichomes. The Trichoderma isolates colonized the glandular trichome tips and formed swellings resembling appresoria. Hyphae were observed emerging from the glandular trichomes on surface sterilized stems from cacao seedlings that had been inoculated with each of the four Trichoderma isolates. Fungal hyphae were observed under the microscope emerging from the trichomes as soon as 6 hrs after their isolation from surface sterilized cacao seedling stems. Hyphae were also observed, in some cases, emerging from stalk cells opposite the trichome head. Repeated single trichome/hyphae isolations verified that the emerging hyphae were the Trichoderma isolates with which the cacao seedlings had been inoculated. Isolates of four Trichoderma species were able to enter glandular trichomes during the colonization of cacao stems where they survived surface sterilization and could be re-isolated. The penetration of cacao trichomes may provide the entry point for Trichoderma species into the cacao stem allowing systemic colonization of the this tissue.