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Spatial variation in the resistance structure of Linum marginale (wild flax) populations to the rust fungus Melampsora lini, and in the racial structure of this pathogen, was investigated by sampling 10 populations distributed throughout the Kosciusko National Park, New South Wales, Australia. Considerable differences were found among populations in the structure of both host and pathogen. Host populations were divided into three broad categories: (1) populations susceptible to all testing races; (2) populations containing a strictly limited number ofresistant phenotypes; and (3) populations with a considerable diversity ofresistant phenotypes. The pathogen populations also showed a range ofdiversity. The major differences between these populations were determined by the occurrence and frequency of four common races of pathogen (races A, E, K., and N). These differences were apparent both at a regional spatial scale (over the 100 km separation of the most distant populations) and at a local scale where major differences were detected between two populations only 300 m apart. The distribution of the four common races of M. lint was consistent with the hypothesis that a fitness cost was associated with unnecessary virulence. In general, however, differences in the structure of pathogen populations from genetically very similar host populations implied that, in addition to host resistance genes, other evolutionary forces are also important in determining the genetic structure of individual pathogen populations.

Is increased support needed for wheat disease research to avert crop losses from current and future strains?

Parasitic damage to the leading shoot of young regrowth eucalypts was found to vary considerably between trees and between different stands but it averaged greater than 20 % 'effective leaf area loss' overall. Many fungal parasites and some of the phytophagous insects responsible for this damage exhibited host specificity or host preference towards trees of a particular subgenus of Eucalyptus. These findings are discussed in relation to the hypothesis that parasites play an important role in the maintenance of stable associations between codominant species of Eucalyptus.

Summary 1.A long-term study (19 years) of a host-pathogen metapopulation involving 133-220 separate populations of the wild plant Filipendula ulmaria and its rust pathogen Triphragmium ulmariae shows marked changes in the occurrence (32-55% demes) and severity of disease and rates of extinction and re-establishment of individual populations (0.006-0.174 and 0.030-0.195 per annum, respectively) over time. 2. Modelling of the spatio-temporal dynamics of disease demonstrated year-to-year changes associated with a range of different environmental features, but also more consistent, longer-term patterns influenced by a complex suite of factors. 3. Both the level of disease and its spatial location varied through time and generated a changing pattern of selective pressure across the metapopulation. 4. Synthesis. Our results suggest that co-evolutionary hot spots and cold spots can be highly dynamic within metapopulations, thereby fuelling the co-evolutionary process even more than previously suspected.