Cacao Diseases: Important Threats to Chocolate Production Worldwide Vascular-Streak Dieback: A New Encounter Disease of Cacao in Papua New Guinea and Southeast Asia Caused by the Obligate Basidiomycete Oncobasidium theobromae

Guest, D., and Keane, P. 2007. Vascular-streak dieback: A new encounter disease of cacao in Papua New Guinea and Southeast Asia caused by the obligate basidiomycete Oncobasidium theobromae. Phytopathology 97: 1654-1657. The basidiomycete Oncobasidium theobromae was identified as the cause of a devastating disease of cacao named vascular-streak dieback (VSD) in Papua New Guinea in the 1960s. VSD now causes losses among cacao seedlings and kills branches in mature cacao trees throughout Southeast Asia and parts of Melanesia. The characteristic symptoms include a green-spotted chlorosis and fall of leaves beginning on the second or third flush behind the stem apex, raised lenticels, and darkening of vascular traces at the leaf scars and infected xylem. Eventually complete defoliation occurs and, if the fungus spreads to the trunk, the tree will die. O. theobromae is a highly specialized, near-obligate parasite of cocoa. It is a windborne, leaf-penetrating, vascular pathogen, and may have evolved as an endophyte on an as yet unidentified indigenous host. The rate of disease spread on cocoa is limited because basidiocarps develop only on fresh leaf scars during wet weather, and basidiospores remain viable for a few hours on the night they are shed. Consequently, very few new infections occur beyond 80-m from diseased trees. Transmission of the disease through seed or infected cuttings has not been demonstrated. Strict quarantine measures applied to the movement of intact plants are crucial in reducing spread of the disease. Integrated management, including the planting of less susceptible genotypes, nursery construction and management, canopy pruning and shade management, provides effective control. HISTORY, DISTRIBUTION, AND IMPACT A devastating disease named vascular-streak dieback (VSD) was distinguished from the various dieback syndromes of cocoa induced by environmental factors and insects in Papua New Guinea (PNG) in the 1960s (6,8). It caused heavy losses of mature trees and seedlings planted near older cocoa. The disease was later shown to be caused by a new genus and species of basidiomycete, Oncobasidium theobromae (5,11). VSD has since been found in most cocoa-growing areas in South and Southeast Asia and PNG, from New Britain in the east to Hainan Island, China in the north and Kerala State, India, in the west. It has been a major problem in the large commercial plantations in West Malaysia and Sabah. It is widespread in Indonesia, including in the fine flavor cocoa plantations in East and West Java, and in the large areas of newer cocoa plantings in Sulawesi. It has also been reported from southern Thailand, Burma, Vietnam, and the southern Philippines. The only known host other than cocoa is avocado, which is also an exotic plant in Southeast Asia and the Pacific (1). It is believed that the fungus evolved on an as yet unidentified indigenous host in Southeast Asia/Melanesia and transferred to introduced cocoa. Thus, VSD is another example of a new encounter disease in cocoa. DISEASE SYMPTOMS AND PATHOGEN BIOLOGY The most characteristic initial symptom of VSD is the chlorosis of a single leaf, usually on the second or third flush behind the shoot apex, with scattered islets of green tissue 2to 5-mm in diameter (5) (Fig. 1A). In some areas (notably Vietnam and Sulawesi) and on some cocoa genotypes, marginal leaf necrosis appears as an atypical leaf symptom (Fig. 1B). Affected leaves are shed within a few days and symptoms progressively develop in adjacent leaves up and down the stem. Lenticels on infected trees usually become enlarged, causing roughening of the bark (Fig. 2). Three blackened vascular traces are visible when the dry surface is scraped off scars resulting from the fall of diseased leaves (Fig. 3). When stems are split, infected xylem is visible as dark streaks within the vascular tissue (Fig. 4). Eventually leaf fall occurs right to the growing tip, which then dies. Lateral buds may proliferate, but then die, causing ‘broomstick’ symptoms to develop (Fig. 5). The fungus may spread internally to other branches and usually kills the tree if it spreads to the trunk. When an infected leaf falls during wet weather, hyphae may emerge from the leaf scar and develop into a basidiocarp. This structure is visible as a white, flat, velvety coating over the leaf scar and adjacent bark (Fig. 2). Basidia develop on the basidiocarp after evening rainfall. Formation and forcible discharge of basidiospores occurs mainly after midnight into the early morning. Basidiospores only remain viable for a few hours into the morning. Basidiocarps remain fertile for about a week on attached branches, but only for a day or two on cut branches. Extended periods of wetness are required for basidiocarp formation, basidiospore release, dissemination, and infection of leaves. Thus, there is a critical link between rainfall peaks and infection periods (4). Corresponding author: D. Guest; E-mail address: [email protected] * The e-Xtra logo stands for “electronic extra” and indicates that the online version contains supplemental material not included in the print edition. Figures 1 to 5 appear in color online. doi:10.1094 / PHYTO-97-12-1654 © 2007 The American Phytopathological Society e-Xtra*