Abstract:
Numerous dead and dying individuals of the Western Cape endemic tree Virgilia oroboides (Fabaceae) were recently
observed within a South African national botanical garden. Root-rot fungi and fungi symbiotic with bark beetles (Curculionidae;
Scolytinae) from diseased trees were assessed for their respective roles in V. oroboides mortality. Disease
progression was also monitored over 1 year. Fungi were isolated from surface sterilized bark and root samples from
diseased trees and provisionally identified using data from the internal transcribed spacer regions (ITS1, ITS2), including
the 5 8S rRNA gene (ITS). Pathogenicity of selected fungi towards V. oroboides was tested under field conditions.
The pathogenicity of various bark beetle-associated Geosmithia (Hypocreales: Hypocreomycetidae) spp. from V. oroboides
were similarly assessed. The only fungus consistently isolated from lesions on the roots and bark of declining
V. oroboides, and never from healthy individuals, represented an undescribed Diaporthe (Diaporthales, Diaporthaceae)
species that was characterized using molecular (using data from the ITS marker and part of the b-tubulin gene, TUB),
cultural and morphological characters. It is an aggressive pathogen of V. oroboides, newly described here as Diaporthe
virgiliae sp. nov. Trees of all ages are susceptible to this pathogen with subsequent bark beetle attack of mature trees
only. All Geosmithia spp. from beetles and/or infected trees were nonpathogenic towards V. oroboides. Diaporthe virgiliae
caused a severe decline in the health of the monitored V. oroboides population over a period of only 1 year and
should be considered as a significant threat to these trees.
