|Ismaiel, Ed - Ed|
Submitted to: Fungal Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2011
Publication Date: 7/23/2011
Citation: Samuels, G.J., Ismaiel, A.A., Rosmana, A., Junaid, M., Cubeta, M., Rodriguez-Carres, M., Guest, D., Mcmahon, P., Keane, P., Purwantara, A., Lambert, S. 2011. Vascular Streak Dieback of cacao in Southeast Asia detection and Melanesia: in planta detection of the pathogen and a new taxonomy. Fungal Biology. 116:11-23. Interpretive Summary: The most serious disease of the chocolate plant in Southeast Asia is known as Vascular Streak Dieback (VSD). This disease is only known from that region and is caused by a fungus. This species only occurs on the chocolate plant and cannot be grown in culture. In the present research we have developed a method to identify the pathogen using a highly specific DNA-based method. Its relationship to other plant pathogenic fungi was determined and, as a result, a new name for the fungal pathogen is proposed. A second, related species was discovered for the first time in cacao in Indonesia. This DNA method allows pathologists to determine whether cacao material is infected with the disease-causing fungus before symptoms appear and thus to prevent the spread of the disease to new areas.
Technical Abstract: Vascular Streak Dieback (VSD) disease of cacao (Theobroma cacao) in Southeast Asia and Melanesia is caused by a basidiomycete (Ceratobasidiales) fungus described in a monotypic genus as Oncobasidium theobromae (syn. =Thanatephorus theobromae). The symptoms of the disease include green-spotted chlorosis and/or marginal necrosis and leaf senescence beginning on the second or third flush of leaves behind the stem apex, raised lenticels, and darkening of vascular traces at the leaf scars and infected xylem. Eventually the shoot apex is killed and infected branches die back and may occasionally lead to death of the cacao plant. Basidia that develop on leaf scars release airborne basidiospores to infect young leaves. The pathogen is known only to infect cacao, an introduced crop; it is suspected to occur asymptomatically in native vegetation but the pathogen cannot be isolated reliably and cultures cannot be maintained. In this study, DNA was extracted from pure cultures of O. theobromae obtained from infected cacao plants sampled from Indonesia and Malaysia. The internal transcribed spacer cluster (ITS), consisting of ITS1, 5.8S ribosomal RNA and ITS2, was amplified, sequenced, and subjected to phylogenetic analysis with Maximum Likelihood and Maximum Parsimony. Results of the phylogenetic analysis show that O. theobromae forms a monophyletic cluster with Ceratobasidium, Thanatephorus, and Uthatobasidium and represents a distinct lineage within Ceratobasidium. Therefore, a transfer of Oncobasidium theobromae to Ceratobasidium is proposed. A PCR-based protocol was developed to detect and identify C. theobromae in plant tissue of cacao. This will allow for early detection of the pathogen in cacao and prevent its spread and will also enable a search for the pathogen in asymptomatic native plants. A second Ceratobasidium species, C. ramicola, identified through ITS sequence, was isolated from VSD-affected cacao plants in Java, and is widespread in diseased cocoa collected from Indonesia.