Severity and changed symptoms of vascular streak dieback caused by Ceratobasidium theobromae in several cocoa (Theobroma cacao) clones in Sulawesi, Indonesia

Vascular streak dieback (VSD), caused by the basidiomycete Ceratobasidium theobromae (syn. Oncobasidium theobromae), is a serious disease of cocoa in Southeast Asia and Melanesia. In the last decade, in many cocoa growing areas in Malaysia and Indonesia, the impact of this disease has increased considerably. In the same period, symptoms of the disease have become dominated by necrotic lesions on infected leaves, in contrast to chlorotic symptoms, characterised by yellowing of the leaf with a typical pattern of remnant green islands, which were formerly ubiquitous. Amplification of basidiomycete ITS and LSU sequences in DNA extracts of VSD-infected plant tissues collected from Sulawesi, Indonesia, and Kerala, India, showing chlorotic symptoms matched previous GenBank accessions (99%). However, neither isolation of an axenic culture nor direct PCR amplification from necrotic leaf tissues was successful. Nevertheless, basidiocarps collected from branches with either necrotic or chlorotic VSD symptom in Sulawesi produced basidiospores and basidia with similar dimensions to those previously documented for C. theobromae suggesting that the same pathogen is associated with both symptoms. Incidence of necrotic symptoms was correlated with disease severity in branches. VSD monitored in a range of resistant and susceptible cocoa clones showed rankings for disease incidence and severity that were consistent for three seasons, confirming that disease resistance was maintained despite the prevalence of necrotic leaf symptoms. A lower proportion of symptomatic leaves on branches 5 months after early symptoms were observed in the resistant Malaysian clone PBC123 (32.6%) compared to the more susceptible M04 (46.9%). Clones common to trials in three different districts in Sulawesi showed a consistently high level of resistance to VSD, including PBC123 and two local selections, M05 and GeniJ.