By Moses Magadza
Windhoek
Scientists in Namibia are considering an extensive national and regional investigation following observations of a threat to the life and survival of the most abundant species of the Acacia tree.
The scientists have observed cancerous outgrowths on Acacia pods which they say are interfering with the reproductive mechanism of the tree, a development they fear can lead to the tree’s extinction in the long term if it is not contained.
The infection has been detected in Acacia karroo, one of approximately 1 300 Acacia species worldwide but the most abundant in sub-Saharan Africa.
This is the first report of unsightly pod malformation on Acacia karroo in Namibia in particular, and in peer-reviewed scientific literature in general.
Studies over several months have shown that the bizarre outgrowths are caused by a highly infectious fungus called Alternaria tenuissima.
The fungus is cosmopolitan and occurs in many hosts including humans, on which it has been known to cause cornea (eye) infections and asthmatic attacks.
The fungus has affected other plant species elsewhere in the world, with disastrous consequences. In the 1970s in India, it wreaked havoc on pigeon peas and several other crops and recently it affected the egg plant – a delicacy eaten as a vegetable throughout the world.
Acacia is a term loosely applied to all thorny, pod-bearing trees in a family called Fabaceae, especially those that are found in Africa. The word Acacia is derived from the Greek akis, which means “a sharp point”.
It was first described from an African specimen (of Acacia nilotica) from Egypt. The Australian sub-genus is Phyllodineae. In Acacia karroo, the fungus causes pod malformation and stops seed production, thus interfering with the reproductive system of the trees.
“It would appear that in Acacia karroo, the infection sets in during flowering. Once the fungus has entered the flower, it triggers non-stop, uncoordinated cell division, just like in a cancerous cell, leading to formation of weird structures, thereby disrupting the proper formation of pods,” plant-microbe biologist Dr Percy Chimwamurombe, who is leading the study, said.
Chimwamurombe said there is still a lot that is not known about this outbreak.
“We still do not know the exact mode of transfer of this fungus from one tree to the other. We suspect insects, but we do not know which ones.
The detailed biology and life history of the fungus are also still unknown. This is important as it will enable us to design appropriate and sustainable control mechanisms,” he said.
He said scientists were also keen to establish the extent of the infection in Namibia and beyond.
“The infection is spreading, and we will contact other experts in the region about it.”
Acasias constitute the highest percentage of tree flora in Namibia and most semi-arid parts of southern Africa. They form the bulk of the diet of browsers that include giraffe, kudu, springbok and large herbivores such as elephants, which have been known to draw tourists and trophy hunters.
Acacia karroo is very widely distributed in Africa and is a prominent component of the savannah ecosystem. It has very nutritious leaves and is also used for medicinal purposes, treating an array of health complications that include sores, headaches, pneumonia, diaorrhea, gynecological problems and eye inflammation. Its sap is sweet and edible.
The bark can be used as a perfume, while its sap is also used as a softener in the tanning industry.
When in flower it attracts lots of insects, notably bees which are crucial to the well-being of the honey industry. In Namibia, the Ovaherero use the sap from its bark to flavour sour milk. Goats and cattle browse on it.
“You can never underestimate the importance of the Acacia karroo. So anything that threatens its existence constitutes a major worry. We are particularly worried by its propensity to jump hosts,” Chimwamurombe said.
Botanist Dr Ezekiel Kwembeya of the National Botanical Research Institute of Namibia said he was also aware of the study, which he said was at a preliminary stage.
He, however, suggested that this outbreak could have been prompted by an imbalance in the ecosystem.
“It is possible that this fungus has been kept in check in the ecosystem but something has gone wrong to cause proliferation. Maybe the fungus has run out of predators and has multiplied. It could be anything; so I agree that there is need for a thorough survey,” Kwembeya said.
Disease-causing organisms have been known to lurk around their targets until they feel that they have gathered sufficient strength and numbers and see an opportunity to pounce – something scientists call ‘quorum sensing’.
Seasoned scientist and Zimbabwe’s first ever Nobel Laureate Professor Christopher Magadza, said Alternaria tenuissima is a “generalist fungus” and infects many plant and animal hosts.
“The most commonly reported mode of dispersal is wind. I am intrigued by reports that this is the first timeǟ