Armillaria mellea in South Africa
Armillaria mellea is a Northern Hemisphere fungus that is parasitic on numerous plant species. The species was once thought to be in Australia but careful research has shown that the Armillaria species found in Australia are quite distinct. As a genus Armillaria is found worldwide and Armillaria mushrooms are typically produced in dense clusters. In 1996 Armillaria root rot was found on oak trees in Cape Town, in the south-west of South Africa and in 1997 mushrooms of Armillaria mellea appeared there. Armillaria root rot had been found on trees in the north of the country and there had been a doubtful 1915 report of Armillaria from the south-eastern coastal area. However, of the verifiable reports of South African Armillaria, none had been of Armillaria mellea and no species of the genus had been recorded in the south-west of the country.
South Africa had been colonized by the Dutch and in the 1600s the Dutch East Indies Trading Company had established the Company Garden in Cape Town and it was in that garden that Armillaria mellea was found. A search of trees in surrounding areas yielded a few infected trees outside, but close to, the Company Garden. Plant genera with signs of Armillaria root rot included Aesculus (horse chestnuts), Albizia (silk trees), Ficus (figs), Hydrangea, Morus (mulberries), Quercus (oaks) and Strelitizia (Bird of paradise flowers). The infected plants were contained within an area of about 380 by 120 metres. Given the numerous stumps it was clear that oaks and other trees in the garden had been dying for many decades.
Samples of fungal tissue were extracted from the mushrooms or from the fungally infected plants and these samples (or isolates) were then grown further in the laboratory. The South African isolates were then analysed in various ways, including DNA comparisons with Northern Hemisphere Armillaria mellea isolates. The DNA analysis showed the Cape TownArmillaria mellea to group close to European isolates, indicating a European origin for the Cape Town infection. Moreover, the DNA analysis showed that all the Cape Town isolates belonged most likely to one genetic individual, or genet. The several hundred metres of infection is therefore very likely to be the result of a single mycelium that has spread through the soil. Given that this Armillaria mycelium was producing mushrooms in the late 1990s it is possible that in future the fungus could be dispersed further by spores.
The authors of the paper listed below believe that Armillaria mellea was introduced into the Company Garden by early European settlers, most likely as a mycelium on a potted fruit tree or grape vine. The Company Garden grew fruit and vegetables for supply to ships travelling between Europe and Asia via Cape Town and historical records show that citrus trees were initially planted within the currently diseased area. The authors were not able to say precisely when the fungus would have been introduced to Cape Town. The data allows only an estimate of between 108 and 572 years as the age of the genet.
If the fungus has been established for so long, why has it gone undetected until almost now? The authors can but say that this is an intriguing question. It may be that the mycelium had been steadily growing for the one or several centuries without mushroom-triggering conditions ever occurring. Alternatively it may be that Armillaria mellea mushrooms had appeared earlier in Cape Town but perhaps so rarely or in such small numbers that nobody had seen them - or perhaps they had not been recognized.