An investigation into the death of native Virgilia trees in the Cape Floristic Region of South Africa

[摘要] ENGLISH ABSTRACT: The Cape Floristic Region (CFR) of South Africa is well-recognised for exceptionally high plantspecies diversity and endemism. However, little attention has been bestowed on the pestsand pathogens in this region, even though these may greatly influence plant distribution andevolution. In this study we identify various arthropods and fungi as pests and diseasecausingorganisms of the ecologically and economically important CFR-endemic tree taxa ofVirgilia. We isolated, identified and determined the pathogenicity of key fungal taxa fromdiseased Virgilia trees throughout the CFR. In addition we evaluated the role of possible pestarthropod taxa, including bark beetles, phoretic mites, larvae of a cerambycid beetle andlarvae of the endemic Leto venus (ghost moth), in the death of Virgilia trees. Key fungaltaxa were identified by comparisons of the internal transcribed spacer rDNA regions of theisolated taxa with those available on GenBank. Pathogenicity of the most commonlyencountered fungal taxa was determined both in the field and under greenhouse conditions.Five different disease symptoms were observed on Virgilia trees throughout the CFR. AtTable Mountain, Virgilia oroboides subsp. oroboides showed symptoms of: (1) several smallcankers on stems, seemingly caused by a Fusarium acuminatum-like fungus, (2) a root rotdisease caused by Armillaria mellea and (3) small bracket fungi on stems associated withSchizophyllum commune. Virgilia oroboides from the Harold Porter National BotanicalGarden was diagnosed with a root disease consistently associated with an un-describedPhomopsis species. Virgilia oroboides subsp. ferruginea and V. divaricata from Knysna andthe Tsitsikamma area often showed symptoms of rapid wilting and death. The Virgilia stemswere damaged by the tunnelling larvae of the ghost moth and those of an unidentifiedcerambycid beetle. Galleries and the surrounding wood tissues often housed theophiostomatoid fungi Ceratocystis tsitsikammensis and Ophiostoma plurianulatum. Theseseem to originate from nitidulid beetles found feeding on gum exudate. Pathogenicity trialsconfirmed the virulence of the undescribed Phomopsis species, the F. acuminatum-likefungus, S. commune and C. tsitsikammensis to Virgilia. All four morpho-species of barkbeetles found in this study, together with phoretic mites on two of the beetle morphospecies,were only collected from dead and dying Virgilia hosts and were classified assecondary pests. Both beetle taxa and mites commonly carried spores of various Geosmithiaspp. These are not pathogenic to Virgilia trees, but may be an important food source for the bark beetles, as it dominated the fungal community in galleries. The phoretic mites wereunable to feed on their Geosmithia associates, but have been observed to feed on dead barkbeetle larvae within galleries. This suggests that the relationship of bark beetles, mites andtheir associated Geosmithia species in this system is complex and in need of further study.Our results show that natural populations of Virgilia play host to numerous destructivepathogens, some of which are non-native (e.g. A. mellea) and a cause for special concern.Additionally, the isolation of the undescribed Phomopsis species and A. mellea frombotanical gardens, with A. mellea now spreading to natural areas, calls for stricter controlover the movement of organic material from these areas.

[发布机构] Stellenbosch University