Testing local cacao selections in Sulawesi for resistance to vascular streak dieback

Vascular streak dieback (VSD) causes serious losses for cacao (or cocoa) smallholders in Sulawesi, Indonesia. The disease is caused by a Cantharellales species, Ceratobasidium theobromae, which colonises the xylem, resulting in leaf chlorosis or necrosis, abscission and eventual branch dieback. The disease is most successfully managed by pruning infected branches and the propagation of resistant genotypes. In participatory trials located in three provinces in Sulawesi, 2.5-year-old trees, which had been clonally propagated from local genotypes or the hybrid progeny of resistant parents, were evaluated for disease severity from 2010 to 2012. Consistent resistance rankings were obtained for clones common to the trials; these were confirmed by re-evaluation in 2014. From plot averages of disease severity, broad-sense heritability was estimated as 0.67-0.92. Two progeny clones, KW617 and ICCRI03, from East Java, had similar levels of resistance in the trials as their respective (resistant) parental clones, PBC123 and Scavina 6. Among four clones monitored for 3 months in West Sulawesi, PBC123 had a higher proportion of healthy leaves on the branch tips and a more restricted spread of infection within the xylem. In contrast, disease symptoms reached the younger leaves in susceptible clones. Individual branches of KW617, monitored from an early stage of symptom development, had a significantly lower number of diseased leaves and higher ratio of new to infected leaves after 9-16 weeks than that in four other clones. Other cacao clones with a relatively high number of diseased leaves during this period overcame infections with the addition of new flushes. Resistance in farm selections did not usually co-exist with yield and bean quality. Deriving new genotypes from crosses between parents with VSD-resistance and high-yield and/or quality traits is required for the production of promising clones with good resistance, yield and quality.