ABSTRACT
Cacao diseases: vascular-streak dieback David I. Guest, University of Sydney, Australia; and Philip J. Keane, LaTrobe University, Australia
1 Introduction
2 VSD symptoms and pathogen biology
3 VSD epidemiology
4 VSD management
5 Case study: the importance of field studies
6 Conclusion
7 Future trends
8 Where to look for further information
9 References
A devastating dieback disease of cacao was first reported in Papua New Guinea (PNG) in the early 1960s (Bridgland et al. 1966a,b, 1967; Shaw 1962). The distinctive pattern of symptom development associated with this new disease differed from the previously known stress and insect related dieback syndromes of cacao (Turner 1967). Characteristic green spotted chlorosis (Fig. 1a) first appeared on a single leaf on the second or third growth flush behind the tip of vigorously growing branches (Keane et al. 1972; Prior 1980). These leaves fell and the chlorosis appeared on adjacent leaves, which also fell, until the entire branch was defoliated or the tip was killed. The symptoms were also distinct from sudden death symptoms characterised by the rapid wilting and death of the entire canopy caused by stem canker (Phytophthora palmivora), Ceratocystis wilt (Ceratocystis cacaofunesta) or root rot fungi (including Armillaria sp., Verticillium dahliae, Rigidoporus microporus, Rosellinia spp. and Phellinus noxius). In the susceptible genotypes of cacao first exposed to the disease in the initial epidemics, the fungus commonly spread to the main trunk and killed mature trees. Infection of the main shoot of seedlings was invariably lethal. The disease was named vascular-streak dieback (VSD) after the characteristic brown streaking in the xylem of infected shoots visible after scraping off the dry surface of the leaf scars left by the abscission of infected leaves. The epidemic caused heavy losses of the mature Trinitario trees that were the common type in PNG at the time and of seedlings planted near older cacao. After the first report of the disease in PNG, it was realised that a devastating dieback disease that had prevented establishment of Amelonado cacao in Malaysia in the late 1950s was this
same disease (Turner and Shepherd 1980) and the disease was found to be common in the new cacao plantings in Malaysia in the 1960s and 1970s. During the initial epidemics in PNG, white basidiocarps were observed commonly on leaf scars following the abscission of diseased leaves during periods of wet weather. The fungus was shown to be a new genus and species of tullasneloid basidiomycete, Oncobasidium theobromae (Talbot and Keane 1971), and was found growing only in the xylem vessels of infected leaves and stems. The pathogen is a near-obligate parasite; unlike the witches’ broom pathogen (Moniliophthora perniciosa) it does not survive in dead branches, and has proven difficult to isolate into pure culture – it emerges readily from infected stem or leaf tissue onto agar media, but is unable to be subcultured and usually does not sporulate in culture. This fungus was shown to be the cause of the disease by inoculation using basidiospores shed from fresh sporocarps collected from cacao plantings (Keane et al. 1972; Keane and Prior 1991). In PNG and Malaysia in the 1960s (and later in Indonesia in the 1980s), the epidemic destroyed many new plantings of cacao that were generally highly susceptible to the disease. Planting seedlings and clonal
Figure 1 Leaf symptoms of vascular-streak dieback in Vietnam: (a) typical ‘green island’ symptom seen commonly until 2004; (b) symptom of marginal leaf necrosis observed commonly in Malaysia and Indonesia (Java, Bali, Sulawesi and Papua) after 2004.
