1Tree Pathology Co-operative Programme, Forestry and Agricultural Biotechnology Institute (FABI), Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, South Africa; 2Environmental Microbiology, Department of Microbiology and Biochemistry, University of the Orange Free State, Bloemfontein 9300, South Africa; 3Mondi Forests, P.O. Box 35, Kwambonambi 3915, South Africa

Background and objectives
The South African Forestry Industry is represented by over 1.5 m ha of exotic plantations. Of these, approximately 50% consists of Eucalyptus spp. of which E. grandis is most commonly planted. In recent years, a number of fungal diseases cause severe damage to eucalypts in South Africa. Early in 1997, a suspected outbreak of bacterial wilt, caused by Ralstonia solanacearum, was reported in the Kwazulul Natal province, South Africa. This pathogen has one of the widest host ranges of all phytopathogenic bacteria with over 50 plant species known to be suscepible [1]. R. solanacearum was first reported on Eucalyptus spp. in the early 1980s in Brazil. Since then, there have been reports of its occurrence on this host in Australia, China, Taiwan and Venezuela. The aim of this study was to identify the causal agent of the wilt symptoms observed on eucalypts in Kwazulul Natal. Three eucalypt clones were compared for their susceptibility to the pathogen.

Materials and methods
An eighteen-month-old clonally propagated E. grandis camadulensis (GC) hybrid in Zululand, Kwazulu Natal plantation, showed signs of wilting. The vascular tissue of infected trees was discoloured and bacterial exudation was produced from the cut surfaces. Bacteria were streaked onto 2% Malt Extract Agar (MEA) and incubated at 25C for three days. The bacteria were purified and identified using the Biolog bacterial identification system. Pathogenicity tests were conducted on three GC clones, namely GC 505, GC 515 and GC 550. Twenty plants of each clone were inoculated with a bacterial suspension. Control plants were inoculated with sterile water. The inoculation technique involved carefully removing the plants from the growth media, clipping the root tips and dipping in a bacterial suspension before replanting.

Results and conclusions
Purified bacterial cultures were identified as those of Ralstonia solanacearum. Four biovars (biotypes) of R. solanacearum have been identified according to acid production from three disaccharides and three sugar alcohols. In Australia and China race I biovar 3 was found infecting eucalypts, whereas in Brazil it was reported to be race I biovar 1 [2]. The biovar of R. solanacearum isolated from eucalypts in South Africa was identified as biovar 3.

After inoculation clone GC 550 showed signs of wilting after three days. Clones GC 505 and 515 appeared to be less susceptible, with fewer plants showing signs of disease. R. solanacearum was re-isolated from all the inoculated seedlings.

The appearance of bacterial wilt on eucalypts in South Africa is of considerable concern to both the Forestry Industry and the Tree Pathology Co-operative Programme. Under optimal conditions R. solanacearum can cause considerable damage. Management strategies to reduce the impact of bacterial wilt is now a priority. Thus, a rapid screening technique to detect this bacterium will be developed in the near future and commercially important clones will be tested to determine their level of tolerance to bacterial wilt.

1. Saddler GS, 1994. Mycopathologia 128, 61-3.
2. Hayward AC, 1994. Bacterial wilt The Disease and its Causative Agent, Pseudomonas solanacearum (eds AC Hayward and GL Hartman) CAB International.