1Department of Plant Pathology, University of Stellenbosch, Private Bag X1, Matieland 7602, Republic of South Africa; 2Tree Pathology Cooperative Programme, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria 0002, Republic of South Africa

Background and objectives

The genus Cylindrocladium has a wide host range and occurs throughout the tropical and subtropical regions of the world. Species are differentiated based on the morphology of the terminal vesicles that extend from the conidiophore stipes, the conidia and their Calonectria teleomorphs. In closely related species a high degree of plasticity is found in some of these characters, which makes identification difficult. A species complex that is beset with taxonomic problems includes C. floridanum, C. scoparium and C. candelabrum. In a recent study employing mating studies, morphology and comparisons at a molecular level, these species were shown to be distinct [1]. Isolates fitting the general morphological criteria for C. candelabrum varied considerably. In the present study, numerous C. candelabrum-like isolates were collected from a wide variety of geographic locations for further characterization. The aim was, therefore, to evaluate the morphological variation among C. candelabrum-like isolates, based on a variety of criteria.

Materials and methods
A sample of 100 isolates from a diversity of geographic locations was selected for study. These isolates were compared using their morphology, sexual compatibility and the nucleotide sequences of their rDNA ITS regions. Isolates were crossed in all possible combinations on Petri dishes containing water agar with sterile carnation leaves (CLA) and incubated at 22C. Morphological comparisons were made on structures derived from CLA plates incubated at 25C under near-ultraviolet light. Several representative isolates of each mating population, reflecting geographic diversity were chosen, for sequence comparisons. The internal transcribed spacer (ITS) regions 1 and 2 on opposite sides of the 5.8 S rDNA gene, in addition to the gene itself, were sequenced and compared.

Results and discussion
All isolates included in this study mated to produce Calonectria teleomorphs with viable progeny. Four distinct mating populations were identified. Based on the results of the initial mating studies, as well as backcrosses done with ascospores from each of the four groups, each mating population was shown to be a genetically isolated, biallelic, heterothallic population. Two of the mating groups appeared to have a sympatric character with isolates collected mainly from South America. The two remaining groups included isolates from diverse geographic locations. All isolates representing the same mating population yielded similar sequences. Small but consistent differences were found between groups. The four groups were also shown to be distinct from species such as C. floridanum and C. scoparium, for which sequence data has recently been published [2]. These results support the hypothesis that distinct biological species are encompassed among isolates that have, until now, been treated as representing C. candelabrum. We expect that a similar situation will also be found in various other species of Cylindrocladium with wide host ranges and geographic distributions.

1. Victor D, Crous PW, Janse BJH, Wingfield MJ, 1997. Systematic and Applied Microbiology 20, 268-285.
2. Jeng RS, Dumas M, Liu FH, Wang CL, Hubbes M, 1997. Mycological Research 101, 285-291.