Dying Clove trees infected by a novel fungus

 

Prepared by Marelize van Wyk: (PhD student)

Clove decline is a serious disease affecting Syzygium aromaticum (clove trees)in North Sulawesi, Indonesia. Yet, the cause of this disease has never been established. During an inspection of the diseased clove trees, at 18 sites, 20-80% of these trees were found to be affected (Fig. A). Diseased clovetrees showed symptoms of wilt, defoliation and vascular staining. Dying trees were typically infested with the woodborer Hexamitodora semivelutina (Fig. B). Larval tunnels are associated with extensive discolouration of the xylem tissue, which had a streaked appearance. Isolations from discoloured wood and larval galleries consistently yielded a fungal species in the genus Ceratocystis. Morphologically, it was similar to the fungus C. fimbriata sensu stricto (s.s), which is a pathogen of Ipomoea batatas (sweet potato). Comparisons of DNA sequence data showed that this Ceratocystis sp. is distinct from C. fimbriata s.s and all other Ceratocystis sp. It could also be distinguished from other Ceratocystis spp. based on colony morphology and a distinct ecology. It was, therefore, described as a new taxon, now known as C. polychroma. The name "polychroma" is derived from the Latin, “multi-coloured” referring to the different colony colours that are observed for this fungus at different temperatures on artificial growth media (Fig. C).

To assess the potential role of C. polychroma in the death of clove trees in Sulawesi, pathogenicity tests were conducted. These tests were both on seedlings in a greenhouse as well as on mature trees in the field. Trees were inoculated with agar plugs bearing C. polychroma or sterile agar in the case of the controls. After 6 weeks, distinct lesions were found on the stems of inoculated trees (Fig. D) and these were absent in the case of the controls. These results lead us to conclude that C. polychroma is contributing to the death of Clove trees in Sulawesi.