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Armillaria Disease in Zimbabwe's National Botanical Garden: A Growing Threat to Plant Diversity
Luke Jimu
Bindura University of Science Education
The National Botanical Garden (NBG) in Harare, the capital city of Zimbabwe stands as a crucial sanctuary for the country’s rich plant biodiversity, playing a key role in conservation, recreation, and education. This expansive garden showcases a diverse array of plant species, including both native and exotic trees that hold significant ecological and cultural value. One half of the garden is dedicated to indigenous plants from Zimbabwe’s woodlands, while other sections feature species representative of the broader African continent, as well as endangered plants from South America, Australia, India, and the Far East. The garden's extensive collection includes over 900 tree and shrub species categorized into distinct areas such as the Zimbabwean rainforest, Highveld Savannah, Zambezi Lowveld Savannah, and Eastern Highlands. Additionally, the NBG is home to the National Herbarium of Zimbabwe, which houses approximately 500,000 plant specimens crucial for research on the flora of the Zambezi region, encompassing Zimbabwe, Zambia, Malawi, Mozambique, Botswana, and Caprivi. However, this vital green haven is currently under threat from the destructive Armillaria disease.
The notorious Armillaria, a fungus genus is wreaking havoc at the NBG, where its rhizomorphs insidiously invade and attack the roots of plants through the soil. This aggressive pathogen has led to widespread host decline and death across the garden. The telltale signs of Armillaria infection include distinctive white mycelial fans visible only when peeling back the bark at the base of infected trees, along with soft decay of the roots. Interestingly, the garden’s climate—characterized by cycles of heavy rainfall followed by dry spells—seems to favour the pathogenic variant of Armillaria, while the less common white rot saprophytic type struggles to thrive. This disease is causing significant damage to the garden’s plant life, disrupting the delicate balance of its meticulously curated ecosystems.
Up to 40 morphologically diverse Armillaria isolates, primarily exhibiting Group III species growth patterns and confirmed through DNA sequencing, have been identified from highly susceptible hosts such as Brachystegia spiciformis, Brachystegia boehmii, Brachystegia glaucescens, Julbernardia globiflora, and various Acacia species. While most infections remain asymptomatic, some affected hosts in the NBG display visible signs of disease, including cankers, resin exudation, and branch dieback, with Acacia polyacantha being particularly affected.
The fight against Armillaria disease at the National Botanical Gardens is far from over. To effectively combat this threat, it is crucial to complement traditional methods—such as fungicide soil treatments, soil excavation, and removal of infected plants—with innovative genomic interventions. Researchers are exploring promising new strategies based on genomic and transcriptomic studies of Armillaria, as well as metagenomic and meta-transcriptomic analyses of soil microbial communities within the gardens. These cutting-edge approaches aim to uncover the role of disease resistance genes and develop targeted solutions to combat the disease, ultimately striving to protect Zimbabwe's Miombo woodlands.
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