Vascular streak dieback (ceratobasidium theobromae): History and biology

Abstract

Vascular streak dieback (VSD) caused by the basidiomycete, Ceratobasidium theobromae (syn. Oncobasidium theobromae, Thanatephorus theobromae), is one of the most important diseases of cacao in the Southeast Asian/Melanesian region, causing branch dieback with infections capable of killing seedlings and mature trees of susceptible cacao varieties. The only known host is Theobroma cacao, but as a new encounter disease it is apparent that the fungus transferred to cacao from an original host, endemic to the region, which so far remains unidentified. Basidiospores that initiate infection are short-lived and dispersed by wind only for short distances. VSD is patchy in distribution but can be severe locally and, recently in Indonesia, has influenced farmers to convert to crops other than cacao. In Malaysia this disease, among other factors, influenced growers to replace cacao with oil palm. The causal pathogen and disease symptoms were first described in Papua New Guinea following a severe epidemic in the 1960s: a unique wind-dispersed basidiomycete pathogen was identified that infected young leaves of cacao seedlings and mature trees, colonizing the xylem and resulting, after about 3 months, in characteristic symptoms of leaf chlorosis, leaf fall, and branch dieback or seedling mortality. Sporulation was observed to occur on the monilioid hyphae emerging from leaf scars. Quite recently, within the last decade, symptoms of leaf marginal and tip necrosis, associated with a longer period of attachment before leaf abscission, have become dominant, replacing the formerly characteristic chlorotic symptoms. As a consequence of delayed abscission, sporulation now frequently occurs on hyphae emerging through cracks in the petiole or leaf mid-rib. The pathogen associated with the newer symptoms, C. theobromae, appears morphologically and genetically identical to the species previously described. However, haplotypes based on ITS sequences have been identified within the region, indicating that some regional genetic variability occurs. Just as in VSD-infected cacao displaying chlorotic symptoms, infections associated with the more recent (necrotic) symptoms spread from initially infected leaves via the xylem to neighboring leaves and, in more susceptible genotypes, reach the tip causing dieback. The two sets of symptoms (chlorotic and necrotic) may occur in the same area and even on the same tree, and their relative frequency is influenced by the season. At higher altitudes and in some genotypes the former symptoms of leaf chlorosis are predominant. Disease severity also decreases with altitude. It remains uncertain whether the recent increase in VSD severity in Sulawesi, East Java, and other areas in the region is linked to the new symptoms. Since the incubation period for the disease is long (3 months or more) it is important to keep seedlings in quarantine for about 6 months before distribution for planting in other locations. As C. theobromae is a vascular pathogen, control by fungicides is difficult, and although effective systemic triazole fungicides have been identified, they are generally too costly for smallholder farmers. The disease is best controlled by sanitation pruning and the adoption of resistant genotypes. Further work is under way to combine resistance with other characteristics such as superior cacao bean quality and yield.