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GENETIC VARIATION AMONG AUSTRALIAN ISOLATES OF PUCCINIA MENTHAEREVEALED BY RAPD MARKERS
GENETIC VARIATION AMONG AUSTRALIAN ISOLATES OF PUCCINIA MENTHAEREVEALED BY RAPD MARKERS
J EDWARDS, PK ADES, DG PARBERY, GM HALLORAN and PWJ TAYLOR
Institute of Land and Food Resources, The University of Melbourne, Parkville, Victoria 3052, Australia
Background and objectives
The mint rust fungus, Puccinia menthae Pers. var. menthae Baxter, causes an economically serious disease of Mentha species grown for essential oil production. The pathogen is highly variable, with many physiologic races identified. In particular, two principal groups of races have been recognised [1]. One group (called spearmint rust) infects M. spicata L. (spearmint) but not M. x piperita L. (peppermint), and the other group (peppermint rust) infects M. x piperita but not M. spicata, yet both groups infect M. x gracilis Sole (Scotch spearmint). Peppermint oil production is an emerging industry in north-east Victoria, Australia, and control of mint rust is a priority, yet little is known about the extent of pathogen diversity in this country. The objective of this study was to examine the background genetic variability within Australian populations of this fungus using RAPD (random amplified polymorphic DNA) markers.
Materials and methods
DNA was extracted from 15 isolates of P. menthae collected from commercial fields and home gardens around Victoria. These isolates from four Mentha species were increased on susceptible hosts from single urediniospores. Genetic variation was assessed using RAPD-PCR with decamer primers. Simple matching distances were computed between the isolates and used to produce a dendogram by UMPGA cluster analysis and a 2-dimensional map of the isolates by non-metric multi-dimensional scaling.
Results and conclusions
113 primers were screened, and 6 revealed differences between the isolates. 64 polymorphic bands were scored from these 6 primers. Both analyses separated the isolates into two distinct groups. One group consisted of the isolates from M. x piperita and the other group consisted of the isolates from M. spicata, M. x gracilis and M. x cordifolia Opiz ex Fresen. (garden mint), corresponding to the peppermint rust and spearmint rust 'races' that were previously identified. Some variability was also evident within the two groups, but there was no overlap between them. The clear separation into spearmint rust and peppermint rust groups is good evidence that the two groups are not hybridising. P. menthae is a highly variable species with several varieties, and variants within varieties, described. Some varieties were initially described as separate species, but later reduced to synonymy with P. menthae [2]. Spearmint rust and peppermint rust, however, come under the one variety, P. menthae var menthae, yet the results from this analysis suggest that this variety consists of two groups that are genetically isolated, therefore perhaps separate biological species. Given the insights now possible using molecular-based techniques, the taxonomy of P. menthae would benefit from re-examination.
References
1. Johnson DA, 1995. Plant Disease 79, 20-24.
2. Baxter JW, 1959. Lloydia 22, 242-246.