Browsing by Author "Machingambi, Netsai"

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    An investigation into the death of native Virgilia trees in the Cape Floristic Region of South Africa
    (Stellenbosch : Stellenbosch University, 2013-03) Machingambi, Netsai; Roets, Francois; Dreyer, L. L.; Roux, Jolanda; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: The Cape Floristic Region (CFR) of South Africa is well-recognised for exceptionally high plant species diversity and endemism. However, little attention has been bestowed on the pests and pathogens in this region, even though these may greatly influence plant distribution and evolution. In this study we identify various arthropods and fungi as pests and diseasecausing organisms of the ecologically and economically important CFR-endemic tree taxa of Virgilia. We isolated, identified and determined the pathogenicity of key fungal taxa from diseased Virgilia trees throughout the CFR. In addition we evaluated the role of possible pest arthropod taxa, including bark beetles, phoretic mites, larvae of a cerambycid beetle and larvae of the endemic Leto venus (ghost moth), in the death of Virgilia trees. Key fungal taxa were identified by comparisons of the internal transcribed spacer rDNA regions of the isolated taxa with those available on GenBank. Pathogenicity of the most commonly encountered fungal taxa was determined both in the field and under greenhouse conditions. Five different disease symptoms were observed on Virgilia trees throughout the CFR. At Table Mountain, Virgilia oroboides subsp. oroboides showed symptoms of: (1) several small cankers on stems, seemingly caused by a Fusarium acuminatum-like fungus, (2) a root rot disease caused by Armillaria mellea and (3) small bracket fungi on stems associated with Schizophyllum commune. Virgilia oroboides from the Harold Porter National Botanical Garden was diagnosed with a root disease consistently associated with an un-described Phomopsis species. Virgilia oroboides subsp. ferruginea and V. divaricata from Knysna and the Tsitsikamma area often showed symptoms of rapid wilting and death. The Virgilia stems were damaged by the tunnelling larvae of the ghost moth and those of an unidentified cerambycid beetle. Galleries and the surrounding wood tissues often housed the ophiostomatoid fungi Ceratocystis tsitsikammensis and Ophiostoma plurianulatum. These seem to originate from nitidulid beetles found feeding on gum exudate. Pathogenicity trials confirmed the virulence of the undescribed Phomopsis species, the F. acuminatum-like fungus, S. commune and C. tsitsikammensis to Virgilia. All four morpho-species of bark beetles 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 as secondary pests. Both beetle taxa and mites commonly carried spores of various Geosmithia spp. 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 were unable to feed on their Geosmithia associates, but have been observed to feed on dead bark beetle larvae within galleries. This suggests that the relationship of bark beetles, mites and their 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 destructive pathogens, 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 from botanical gardens, with A. mellea now spreading to natural areas, calls for stricter control over the movement of organic material from these areas.