Abstract:
Mangroves are critically important components of coastal ecosystems. However, their survival is globally threatened, mostly due to impacts resulting from human activities. Reports of mangrove deaths associated with pathogens and insect pests have emerged during the past few years. In South Africa, mangrove species are under pressure from both environmental and anthropogenic disturbances, potentially making them more susceptible to diseases. We present the most detailed evaluation of possible biotic causes of mangrove decline in South Africa to date. Surveys covering the entire distribution range of mangroves in the country were conducted. Qualitative and quantitative data from siltation of pneumatophores, stand density, diameter at breast height and the presence of wood-boring beetles were correlated with disease incidence and severity to elucidate the possible relationships with mangrove health. Phylogenetic analyses were performed to determine the taxonomic placement of fungi isolated from symptomatic trees. Of five true mangrove species and two mangrove associates examined, only Avicennia marina showed signs and symptoms of branch and stem cankers, die-back, wood-boring insects and leaf galls. Barringtonia racemosa showed symptoms of fruit and leaf disease and Hibiscus tiliaceus was observed with herbivory by leaf-feeding beetles. Using a multivariate approach, the presence of beetles and high pneumatophore siltation appeared to be associated with the observed die-back and canker levels of A. marina. Four main fungal groups were recovered from symptomatic trees. The results suggest that natural and anthropogenic stressors exerted on the mangrove trees lead to the colonization of an array of opportunistic pests and diseases.
