particularly prevelant along waterways and on roadsides. Often it is associated with bracken fern, it can quickly over-run large areas. Under favourable conditions it is capable of spreading in pasture. In recent years blackberries have appeared not to be as vigorous in some areas of the region, possibly due to the effects of the blackberry rust disease.
It is believed that the blackberry leaf rust fungus was imported into Tasmania illegally and may not be as vigorous as the rust strains in Victoria. Alternatively, the blackberries growing in Tasmania may be of a different genotype to those growing in other states of Australia and as a result, this Tasmanian blackberry genotype could be less susceptible to blackberry leaf rust disease.
Biological control using the leaf rust fungus Phragmidium violaceum, has been achieved with spectacular success at some locations. At Foster in south-eastern Victoria, for example, large well established blackberry infestations have been defoliated sufficiently to allow native plants to grow through the blackberry. At other locations, however, rust disease is having little of no impact on blackberry biomass.
Rust fungi are desirable biocontrol agents because of their host specificity and the fact that their spores can be spread thousands of kilometres by the wind. While it is desirable that rust strains do not cause disease of commercial berry crops, it is this host specificity factor which may be reducing the effectiveness of biological control. CRC personnel on a field trip in Western Victoria in December 1997 came across two thickets of blackberry growing adjacent to each other on the side of the road. The two thickets were different types of blackberry as one had an erect growth habit and other more prostrate. The blackberry with the prostrate growth habit was more severely infected with rust disease that that found on the erect blackberry. Similar difference in disease severity within a particular infestation have been observed in the Strezlecki ranges, Victoria.
The consequence of this differential biological control is that one blackberry type may simply be replaced by another type. Given that blackberry plants at a particular site are growing under similar environmental conditions it was considered that the difference is disease levels would be due mainly to genetic factors. Other factors that may be affecting the level of disease include canopy microclimate (rust spores require water to germinate) and/or the proportion of susceptible leaf tissue available for infection (young leaves are more susceptible to infection that old leaves). To explore this further the taxonomy of European blackberry had to be characterised and their susceptibility to rust strains now present in Australia determined.
Rust disease was first observed in eastern Victoria in 1984, presumably as a result of an illegal introduction of one or more rust strains. An additional strain, F15, was widely released in temperate Australia in the summers of 1991 and 1992. Strain F15 was released because it was expected to be more effective than the "illegal" strain/s, especially on the most common blackberry, R.discolor. The presence of the illegal rust strain/s and the fact that these strains may have "crossed" with F15, means that the genetic structure of the current rust population is unknown.
The genetic structure of the blackberry population is also poorly understood. The taxonomy needs a major revision in order to achieve a consensus among the States in their names and descriptions. In order to overcome some of the taxonomic difficulties researchers are determining the genotypes of individual plants by DNA fingerprinting. Some blackberry taxa, such as R.ulmifolius, are compromised of several genotypes, whereas our most common taxon, R.discolor appears to exist as a single genotype throughout Australia. Research which will sort out these taxonomic problems is being conducted in close association with Dr David Symon of the State Herbarium, SA, and Dr John Hosking, NSW Agriculture. R.erythrops in South Australia, for example, appears to be synonymous with R.rosaceus in Victoria, because both plants tested have the same DNA fingerprint. It is anticipated that this collaboration will result in a comprehensive revision of Rubus taxonomy in Australia, resulting in u seful and accurate keys for weed managers.
While the DNA fingerprinting work continues, experiments have commenced to identify blackberry genotypes which are resistant to a particular rust strain present in Australia. Strain F15 is being used as a reference strain in all studies and three strains of rust have been isolated from diseased blackberries at three different sites; eastern Victoria, western Victoria and Adelaide, SA. A blackberry garden has been established of all the genotypes identified to date. At least 25 blackberry genotypes will be inoculated with the three "wild type" strains and strain F15 under optimum and controlled environmental conditions. Assessment of disease symptoms should identify any Rubus genotypes which are resistant to the rust disease. Any differences in disease response will also provide the first evidence that different strains exist in the Australian rust population. If disease resistance is identified in any of the blackberry genotypes, then additional strains of P.violaceum, which can multiply rapid ly on the resistant genotypes, could be identified in Europe and introduced as biological control agents.
Whereas genetic resistance may be the factor most limiting rust epidemics in summer rainfall areas, a susceptible blackberry taxon growing under summer drought conditions may not experience the microclimate conditions, such as a dew period, required for disease development. In Adelaide for example, a week of temperatures in the range 35-40 degrees Celsius, appears to halt the production of the rust spores responsible for repeated cycles of disease.
The next phase of research, therefore should be to quantify and model all those environmental factors responsible for the successful establishment and rapid spread of rust disease. While temperature and leaf-wetness duration are likely to be important factors explaining infection efficiency, disease severity might also be explained by environmental factors that affect the rate of blackberry growth and thus the proportion of susceptible leaf tissue that is available for infection. In addition, environmental stress factors may be inducing disease resistance in leaves that would normally be susceptible under optimum growing conditions for the blackberry.
The development of this knowledge base could then be used for devising novel methods for overcoming the environmental factors limiting disease development. This might include the modification of local humidity levels at key times or altering the proportion of susceptible blackberry tissue and/or canopy architecture by use of fire, slashing, grazing or other means. It could mean increasing the disease susceptibility of the blackberry by applying chemicals that alter the biochemistry (defense response) of the plant.
The end result will benefit the land manager in the battle with blackberry and the research has certainly highlighted the fact that nothing is ever simple as it seems.