1Tree Pathology Cooperative Programme, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria, 0002, South Africa; 2Department of Plant Pathology, Iowa State University, 351 Bessey Hall, Ames, IA 50011, USA

Background and objectives
A. mearnsii (black wattle) is the third most important forestry species in South Africa and is especially popular among private farmers. The tree is versatile in that the wood is used in the paper and pulp industry, while extracts from the bark are used for leather tanning and the manufacture of adhesives. Ceratocystis albofundus is a newly described species causing a serious disease of A. mearnsii in South Africa [1]. The disease is known as Ceratocystis wilt and is characterized by xylem discolouration, cankers, wilt and die-back of affected trees, similar to diseases caused by the close relative C. fimbriata. The wattle pathogen is highly virulent and is capable of killing susceptible trees within 6 ;weeks; thus, it is of major concern to the wattle industry.

C. albofundus is known only from South Africa and is also thought to occur on indigenous Protea spp., so it has been suggested that the pathogen is endemic to South Africa. The aim of this study was to determine the genetic diversity of C. albofundus in South Africa. This information might suggest whether the pathogen is indigenous or introduced.

Materials and methods
Isolates of C. albofunduswere collected from plantations in the major commercial wattle growing areas of South Africa and also from naturally established weed stands growing outside the commercial areas. Total genomic DNA of these isolates was extracted and digested with various enzymes before electrophoresis. Nuclear DNA fingerprints were made by probing PstI digests with the 15 ;bp oligonucleotide probe, (CAT)5 [2]. Mitochondrial DNA fingerprints were obtained by digestion of total DNA with HaeIII. Both the gene diversity, H, of the South African population and the genetic distance among isolates were determined.

Results and conclusions
A high level of gene diversity was found for both the nuclear and mitochondrial genome. Values obtained for C. albofundus were compared with those for C. eucalypti, C. virescens, and Chalara australis, which have three different reproductive strategies, namely obligate outcrossing, selfing and asexual reproduction. Values for the mitochondrial diversity for C. albofundus were higher than those of the other species. Values for nuclear diversity suggests an active sexual state for this fungus, with values comparable to the obligate outcrossing C. eucalypti. These data, together with its apparent occurence on native Protea sp., support the hypothesis that C. albofundus is indeed endemic to South Africa.

1. Wingfield MJ, De Beer C, Visser CD, Wingfield BD, 1996. Systematic and Applied Microbiology 19, 191-202.
2. DeScenzo RA, Harrington TC, 1994. Phytopathology 84, 534-540.