Modern approaches to species concepts in downy mildews

Inadequate species definitions present a serious problem to the pathologist working in plant hygiene or quarantine which demands urgent attention. Species concepts in the downy mildews have not kept pace with developments in evolutionary and molecular biology, or with advances in ecological genetics, because the downy mildews are obligate biotrophs that are not easily cultured in the laboratory. Existing approaches to species concepts in this group (morphometric, Gaumann's 'biological species' and Skalicy's 'eco-physio-phentic' concepts) are examined and found to be inadequate and potentially misleading. The systematic treatment of the downy mildews is beginning to benefit from the application of modern methods of systematic analysis. The contribution and potential of ultrastructure, karyotyping, sterol and fatty acid composition, isoenzyme patterns, molecular biology, numerical methods, immunoassay and hypotheses of coevolution to the development of species concepts are reviewed and their wider application is seen as a priority. The application to the downy mildews of two widespread species concepts, the biological and phylogenetic concepts, is examined in the light of the information gained from modern methods of analysis, but neither is found adequate to describe species of downy mildews as they occur in nature. Modern methods can suggest phylogenetic relationships on the basis of statistical probabilities and may also detect microevolutionary change, but it is concluded that much more information is required about individual breeding systems, gene flow, ecology, phylogeny and distribution before informed decisions about the delimitation of most species can be made. Until patterns of genetic diversity can be established, a modified phylogenetic species concept may offer one interim solution to the problem of species definition in the downy mildews.