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Wednesday, March 29, 2017

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FABI Articles : Large scale die-off of an Iconic South African Tree – Euphorbia Ingens

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Large scale die-off of an Iconic South African Tree - Euphorbia Ingens


 Unexplained die-off of Euphorbia ingens (Candelabra, Naboom or Mokgoto) in the Limpopo Province (Fig. 1) has captured the attention of researchers associated with the CTHB. Results from research on this phenomenon suggest that it is the product of a complex interaction between various biotic and abiotic factors. Discoveries made by Johan van der Linde for his M.Sc. dissertation laid a solid foundation towards a better understanding of the large-scale deaths of these iconic trees. His research also led to the description of two previously unknown fungal species and an increase in our knowledge of the fungal species and an increase in our knowledge of the fungal biodiversity of South Africa. During the last 10 years it has been observed that E. ingens trees in the Limpopo Province are becoming diseased and dying without direct evidence of a causal agent.

 

Mortality of E. ingens


A survey conducted in 2006 by Mrs. Renita Malan at the National Zoological Gardens Biodiversity Conservation Centre (NZG) in Mokopane (Potgietersrus) revealed that all E. ingens trees in this centre were diseased, and many had died over a short period of time. A pilot study conducted in 2007 by Jolanda Roux (Professor in TPCP/CTHB), aimed at characterizing disease on E. ingens at the Game Breeding Centre in Mokopane, identified several fungi and insects associated in dying trees. A grey discoloration of the tree stems and branched, as reported previously by Malan (2006), was also found. Additionally, rotting and browning of the succulent branches, white and yellow spots in the main stems and numerous insects infesting the stems and branches were found (Fig. 1).

Studies conducted during 2009 and 2010 at multiple sites in the Limpopo Province considered both abiotic and biotic factors in the death of E. ingens trees. Three main areas showing trees suffering from die-off were studied: The National Game Breeding Centre in Mokopane, a domestic farm in the vicinity of the Capricorn Toll Plaza and a private game farm close to Louis Trichardt. These areas were compared with two healthier sites located in the North West Province. As a first step the disease symptoms on the trees were characterized. Various spots and scars on the external areas of the succulent branches, internal rotting of the succulent branches and grey discoloration of the healthy succulent branches were observed (Fig.1).

A high diversity of fungus was isolated from diseased E. ingens and a number of insects belonging to different genera were collected. The most commonly isolated fungi resided in the Botryosphaeriaceae, Cordycipitaceae, Microascales, Nectriaceae, Ophiostomataceae and the Teratosphaeriaceae. In the Botryosphaeriaceae, Lasiodiplodia mahajangana and Lasiodiplodia theobromae were identified. The Microascales included a previously unknown species, which was described as Gondwanamyces serotectus. Insects included Cyrtogenius africus (Coleopters: Curculionidae, Scolytinae), Cossonus sp., Stenoscelis sp. (Coleoptera: Curculionidae, Cossoninae) and a Megasis sp. (Lepidoptera: Pyralidae).

Temperature and rainfall studies showed an increase in temperature with reduced rainfall in areas where E. ingens displayed severe die-off. Temperature and precipitation data were analysed over a 40-year period at four sites in the Limpopo Province and two sites (areas of less severe die-off) in the North West Province. Compared to the Limpopo Province, the sites in the North West Province were cooler with higher rainfall levels over the 40-year period. Furthermore, evapotranspiration and water balance levels were investigated and results showed that trees were under more stress in the Limpopo Province. This finding might explain the more severe die-offs in the Limpopo Province since the trees will be more vulnerable to pathogens and insect attack due to their state of increased stress.

The death of E. Ingens appears to be caused by a combination of biotic and abiotic factors acting in concert. Further studies are needed to consider the unexpected die-off of different species of Euphorbia residing in different provinces. These studies must also include more in depth evaluation of land management and fire regimes as well as climate data for each province in order to better understand the possible involvement of fungi, insects and the various abiotic factors in the decline of native trees in South Africa.    


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