Alan R. Wood
ARC-PPRI
Private Bag X5017
Stellenbosch
7599
South Africa

Email: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

During the 1980’s a dieback disease of the invasive alien weed Rooikrans was noted in the Groot Brak (Mossel Bay) area in the southern Cape. 

The plants very rapidly die back, the leaves senescing and dropping together (Fig 1). Field observations indicate that this fungus is now distributed from the Cape Peninsula eastwards to Humansdorp. In certain areas a high proportion of this weed have been killed (Fig 2, 3), though field observations indicate that indigenous plants in the immediate vicinity are not affected.

Unfortunately early attempts to determine the etiology of this disease were unsuccesful. Then in 1995 and 1996 a basidiomycete was isolated from diseased plants which proved to be highly pathogenic to Rooikrans, and which caused high mortality rates when inoculated into mature plants in the field. This fungus has recently been described as Psuedolagarobasidium acaciicola (Wood and Ginns 2006).

The fungus is a wood-rotting root pathogen, and appears to spread by root-to-root contact, as well as basidiopores. The latter are not readily produced, but a purple hymenium can be found on occasion amongst the leaf litter and soil at the base of the plant’s stem (Fig 4). This low spore production may explain the relatively slow spread of this fungus amongst and between populations of this weed. Early inoculation studies were done using the fungus grown on moist sorghum seed which was inserted into holes drilled (using a cordless power drill) around the circumference of mature trees (a hole about every 5 cm around the circumference) through the bark and into the wood. Although this method is highly successful, it is limited by the battery life of the drill used. Subsequently it was found that an aqueous suspension of mycelium which had been grown in liquid culture, and inoculated into small wells made in the tree stem base using a chisel and mallet, was equally effective to inoculate trees.

Although Rooikrans is the most widely distributed and abundant weed in the Western Cape Province, having invaded vast areas of strandveld and limestone fynbos, it is also the most sought-after fuel wood in these areas. Because of its desirability as a fuel wood for braais, pizza ovens and stove fuel, it fetches the highest price in the wood market in the Cape Town region and is therefore an important source of income for many people. Due to this conflict-of-interest, no vegetatively destructive biological control agents may be introduced against Rooikrans. Thus only agents that reduce the seed production may be used. A seed feeding weevil, Melanterius servulus, and a flower-bud galling midge, Dasineura dielsi, have been introduced from Australia. These are both now established and significantly reduce the seed production of Rooikrans.

The Rooikrans dieback fungus, P. acaciicola, may therefore prove to supplement the biological control agents used against the weed in South Africa. Because it is indigenous, already present in many Rooikrans stands, and has a slow dispersal rate, it therefore has the potential to be manually distributed using a simple and cheap technology in areas where the plant is unwanted or causing economic damage (nature reserves and natural grazing land). Areas where the weed is harvested for wood may be left alone. Trials to determine whether this potential can be met have been set up in the Agulhas plain, and will be monitored into the future.

Parts of this work have been supported by funding from the Directorate of Natural Resources (NDA), the Working-for-Water Programme (DWAF), and the Agulhas Biodiversity Initiative which are gratefully acknowledged. Jim Ginns (Canada), who identified and decribed the species despite being recently retired, is also gratefully acknowledged

Reference
Wood AR, Ginns J (2006) A new dieback diseae of Acacia cyclops in South Africa casued by Psuedolagarobasidium acaciicola sp.nov. Canadian Journal of Botany 84: 750-758.