Abstract
Vascular-streak dieback (VSD), caused by Ceratobasidium theobromae (Tulasnelloid; Basidiomycetes), is a new-encounter disease of cocoa with a significant impact on the industry in Indonesia, Southeast Asia and parts of the Pacific. The pathogen can defoliate mature trees and kill young trees. The typical characteristic chlorotic, green spotted leaf symptom was discovered in PNG 1960s, however new symptoms of leaf necrosis were reported across the distribution of the disease in the mid-2000s. Both symptoms can exist in the same branch. A preliminary study suggested that two pathogens may be associated with these different symptoms, but this was based on a small number of observations. The current study aims to explain why leaf symptoms vary, and to identify the as yet unidentified original host. Hierarchical sampling supported by GPS mapping collected 1,500 petioles from chlorotic and necrotic symptomatic to asymptomatic leaves across Sulawesi. DNA was extracted and pathogen presence confirmed using primers specific for basidiomycetes; forward (ITS1F: 5-CTTGGTCATTTAGAGGAAGTAA-3) and reverse primers (ITS4B: 5-CAGGAGACTTGTACACGGTCCAG-3)] then for C. theobromae; forward (Than_ITS1: (5’-GAGTCTTGGCAGTTGCTG-3’) and reverse primers (Than_ITS2: 5’- AGAAGCGGTCATCTGTA-3’). The Nucleic acid sequence variation will be determined using SSR (microsatellite) analysis of ITS sequences, and variation linked to sampling location and symptom type.