Biological Control
Background
The blackberry leaf-rust fungus (Phragmidium violaceun) has been in Australia since the mid-1980’s following an unauthorised introduction. In the early 1990's a strain of the rust from France (referred to F15), which had been thoroughly tested in Europe, was approved for release by the authorities.
Since these introductions the rust has been successful in controlling European blackberry (Rubus fruticosus agg.) in some but not all areas where climatic conditions for disease development are optimal. The high genetic diversity of European blackberry in Australia may have contributed to the limited success of the rust in some areas.
Work in the 1990s by the Cooperative Research Centre for Australian Weed Management (Weed CRC) led to the identification of at least 14 different taxa and 40 different genotypes of European blackberry in Australia, which vary in their susceptibility to the rust population that existed at the time.
Release of Eight Additional Strains of the Blackberry-Leaf Rust
Starting in 1999, CSIRO and the Weed CRC, with financial support from the Department of Agriculture and Food Western Australia (DAFWA) and the Department of Environment and Conservation (DEC), set up a trap garden of Australian blackberry genotypes at the CSIRO European Laboratory in Montpellier, France to source additional strains of the blackberry leaf-rust fungus that can infect the wide range of taxa and genotypes present in Australia.
Eight strains were collected, purified and then imported into the CSIRO Black Mountain Containment Facility for testing before approval was obtained in early 2004 to release them in the environment. The rational to introduce additional strains of the rust in Australia is to increase genetic diversity in the population to enhance the rust's capacity to adapt to its variable host and different environmental conditions. This should lead to the rust having a greater impact on European blackberry across the Australian landscape, which will assist in containing present infestations and reducing spread.
A 3-year project building on previous research was established in June 2006 to coordinate the national release of the eight additional strains of the rust. The project was a partnership between CSIRO (the lead agency), the Victorian Department of Primary Industries, the University of Tasmania and the community, with financial support from the Australian Government Department of Agriculture, Fisheries and Forestry under the Defeating the Weed Menace Initiative.
A total of 379 releases were made by participants in NSW, ACT, WA, SA, Qld and Tasmania, who received release kits distributed by the project via post or at field days (Figs 1, 2). Each kit contained spores of the various strains, which had been mass-produced at the CSIRO facilities, guidelines and other necessary material to make a release. Of the 224 participants who provided feedback on their release, 96% confirmed that rust symptoms had developed on inoculated canes soon after the release indicating that our release protocol was straightforward and successful. A total of 110 releases were made in Victoria by colleagues from the Department of Primary Industries, in collaboration with the community.
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Figure 1. Locations in Western Australia and Tasmania where all eight new strains and strain F15 of the blackberry rust fungus were released between June 2006 and June 2009.
Grey circle = Infection from at least one of the strains seen on inoculated canes after the release;
Open circle = No feedback received after the release;
Cross = No sign of infection after the release.
Figure 2. Locations in eastern states where all eight new strains and strain F15 of the blackberry rust fungus were released between June 2006 and June 2009. (Note: In most instances in Victoria, only one of the new strains was released at a site. Also not all release sites in Victoria are shown on this map). Grey circle = Infection from at least one of the strains seen on inoculated canes after the release; Open circle = No feedback received after the release; Cross = No sign of infection after the release.
Symptoms of the leaf-rust fungus on the upper side of
blackberry leaves.
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Symptoms of the leaf-rust fungus on the under side of
blackberry leaves.
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Mass-production of spores of the blackberry leaf-rust fungus on detached leaves. |
Field inoculation of blackberry with the rust fungus.
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Post-Release Research on the Additional Rust Strains
Following the release of the additional strains, the impact of the rust was measured on blackberry using fungicide exclusion techniques in the field and on standardised potted plants in a shadehouse. Although limited amount of disease developed in field experiments due to recent drought conditions, plants sprayed with water produced significantly less fruits than fungicide-sprayed plants at some of the sites. Similarly, in one of the shadehouse experiments above and below-ground biomass was significantly reduced in plants sprayed with water only.
It is not possible to distinguish between strains of the blackberry leaf-rust fungus using morphological characters. Consequently reliable and robust molecular markers were developed to monitor establishment and persistence of the additional strains after release at four representative sites in NSW and Victoria. Alleles specific to the additional strains were found in 12.7% to 100% of rust samples collected at these sites 1 year after release. After 2 years, analyses indicated that rust populations sampled from two sites in NSW retained 44 and 86% ancestry to the new strains. This confirms that genes of the additional strains were successfully incorporated into the local existing rust populations, a first step towards increasing the adaptation potential of the rust fungus to improve biological control effectiveness.
Is There a Need to Redistribute the Blackberry Leaf-Rust?
A total of 489 releases of the additional eight rust strains were made in areas with major infestations of European blackberry across Australia during the 2006-09 national project (Figs 1-2). The additional strains, or new recombinant genotypes, should spread naturally to other blackberry infestations within the region where they were released and established. Spores of the blackberry-leaf rust are spread by wind and can travel long distances in relatively short periods of time. Given suitable climatic conditions, the rust genotypes most adapted to particular areas should adversely impact blackberry populations over time through regular defoliation of plants, pending climatic conditions are suitable for disease development, but will not eradicate them.
The redistribution of rust-infected blackberry material from one site to another by land owners will not necessarily assist the spread of the additional strains or new recombinant genotypes since it is not possible, without sophisticated molecular tools, to determine if the material transferred contained genes of the additional strains.
Is the Blackberry Leaf-Rust Safe?
Rust spores are not toxic to humans and animals but may cause irritation to people sensitive to pollens, dusts etc. In such cases, it is recommended to wear safety equipment such as goggles, a respiratory mask and gloves when handling rust spores or infected foliage.
The additional rust strains of the blackberry leaf-rust fungus were thoroughly tested before approval was obtained to release them in the environment. They were shown in host-specificity tests to be highly specific towards weedy European blackberries. Results from these tests also demonstrated that that these additional strains pose no greater risk to commercial blackberry cultivars and Australian native Rubus species than the strains that already existed in Australia before their release.
There are a number of fungi that cause 'leaf spots' on cultivated blackberry: Cercospora rubi (causing Cercospora leaf spot), Elsinoë veneta (causing Anthracnose), Septoria rubi (causing Septoria leaf spot) and the rust fungus Kuehneola uredinis (causing cane and leaf spot). It is important for growers to be able to distinguish these leaf spots from those caused by the blackberry leaf-rust, the rust fungus introduced for the biological control of weedy blackberry.
Leaf-spot diseases may be causing isolated incidents of reduced fruit yield, but the contribution of the blackberry leaf-rust in relation to other fungi that cause leaf spots is probably negligible and should be determined in each instance. If rust occurs on raspberries, then it is most likely caused by Phragmidium rubi-idaei, a species related to the blackberry leaf-rust.
For the last three decades, exotic rust fungi have been used for the biological control of weeds in Australia without even an unpredicted shift between forms of the same host species. Their safety record is unblemished, indicating that the scientific guidelines followed prior to introduction of the fungi were appropriate. The spectacular success of some of these fungi in reducing the impact of weeds, as seen with Puccinia chondrillina introduced in the early 1970's for the control of skeleton weed and more recently with the bridal creeper rust (see Biological Control of Weeds. The Process PDF 2.05 MB), reminds us that there is a lot to be gained from this method of weed control.
See also General information on biological control.
What to Expect from Blackberry Biological Control
There is no single solution to deal with the blackberry problem in Australia. Consequently any improvement of biological control through the release of additional rust strains will be most welcome, particularly at sites where implementation of other control methods is inappropriate or impractical. However, biological control is not a silver bullet, but rather a complement to existing control techniques.
Biological control will not be effective across the range where blackberry infestations occur in Australia and therefore an integrated weed management approach will be necessary. For example, the blackberry leaf-rust fungus will not have a major impact in areas where the growth rate of blackberry and infection potential of the rust are limited by sub-optimal environmental conditions such as high temperatures and summer drought. The leaf-rust fungus causes most severe disease on emerging blackberry leaves during active shoot growth; older leaves on shoots have been shown to be more resistant to the rust.
Recovery of huge areas of land for the production of pasture and the establishment of seedlings in plantation forests is expected to follow effective control of blackberry. In areas reserved for conservation of native flora and fauna, control of blackberry infestations is a first step towards achieving an increase in species diversity. However, reinvasion by weeds is a recurrent challenge and revegetation strategies should thus be considered in tandem with any weed control program. The recreational and aesthetic value of reclaimed land for activities such as bushwalking, fishing and bird watching will be improved significantly following blackberry control, and stock, fauna and people will have improved access to waterways.
For more information see publications list. |
