By Richard Black
BBC science correspondent
Scientists in southern Africa say they are mystified by a disease which appears to be killing baobab trees.
Their trunk widths are exceeded only by the sequoias of North America
Farmers have been reporting blackened and dying trees for several years and this has now led to an investigation.
Prof Mike Wingfield of the University of Pretoria says a fungus is probably involved but far more research is needed to explain the disease fully.
Baobabs are famous for their extremely bulbous trunks and their ability to live many years in drought-hit areas.
"Farmers have come to us and said, 'you know, our baobabs are dying'," said Prof Mike Wingfield, the director at Pretoria's Forestry and Agricultural Biotechnology Institute.
"And a year ago, we decided to make a survey, to try to find baobabs that people were saying were dying.
"We had people go out and climb these trees and take samples. And we found some fungi; we've even gone as far as to inoculate this fungus into trees to see if we can kill them.
"But the results are not conclusive - I don't believe that we've really got to the bottom of it."
One theory is that the fungus only takes hold when a tree is already damaged.
"One theory is that elephants are damaging them," Professor Wingfield said. "I think elephants eat on, and damage, the stems of the trees... and then these secondary organisms start to infest them."
Baobabs are an extremely unusual type of tree - more like a cactus in their ability to survive drought by storing water in their enormous trunks - and are reputed to live for several thousand years.
From a research point of view, they are actually quite difficult to work with.
"They're generally - the ones that are supposed to be dying - very big; and the baobab is like a succulent, it's not like a typical tree. So if you take a chainsaw to it, it just spews out a whole bunch of water - it's sort of mushy inside."
Their value to many communities in Africa is immense, but Prof Wingfield fears the trees' broader commercial value may not be on the scale required to draw further funds to mount a full-scale study of what is attacking them.