Mysterious tree disease of the 70s and 80s unmasked

A mystery disease hit South Africa's pine trees 40 years ago. New DNA technology has found the killer, say Andi Wilson, postdoctoral fellow at the University of Pretoria, Brenda Wingfield, previous vice-president of the Academy of Science of South Africa and DSI-NRF SARChI chair in Fungal Genomics and professor of genetics at the University of Pretoria (UP), and Prof Michael John Wingfield, adviser to the executive at UP.

In the 1970s and 1980s, pine trees growing in various forestry plantations in the Western Cape began to die in patches. These trees succumbed to a mysterious root disease and the patches expanded gradually. Spontaneous regrowth of seedlings in the patches died dramatically.

As in many other true-crime dramas, the finger was initially pointed at the most likely suspect: the root-infecting Phytophthora cinnamomi. Its name – plant (phyto) destroyer (phthora) – reveals its power to cause harm; the pathogen is known to cause disease in almost 5 000 different plants.

After further investigation and the collection of many samples, tree pathologists shifted the blame onto the fungus Leptographium serpens (now known as Leptographium alacre). This fungus is well known to be transported by insects and was previously only known in Europe. It was visually identified from the roots of the dying trees. Now it was the prime suspect.

Doubts lingered, though. Most Leptographium species are not known to act as primary disease agents and so L. serpens was most likely not able to cause the disease. Other fungi were also found within the roots of the diseased trees but could not be identified at the time due to a lack of more advanced techniques.

Knowing that the then-available technologies could not provide the complete answer to this mystery, the pathologists took more samples from the dead and dying pine trees, and stored them carefully. The hope was that one day they would have a better idea of the cause of this disease outbreak.