5.2.5
HOW EFFECTIVE IS PHRAGMIDIUM VIOLACEUM AS A BIOCONTROL AGENT OF BLACKBERRIES IN AUSTRALIA?

VKJ EVANS1, E BRUZZESE2, F MAHR2 and RT ROUSH1

1Department of Crop Protection and Co-operative Research Centre for Weed Management Systems, University of Adelaide, Waite Campus, PMB 1, Glen Osmond, South Australia 5064, Australia; 2Department of Natural Resources and Environment, Keith Turnbull Research Institute, PO Box 48, Frankston, Victoria 3199, Australia

Background and objectives
European blackberry (Rubus fruticosus aggregate) is an important weed of natural and agricultural ecosystems in Australia. Disease symptoms, caused by the leaf-rust fungus Phragmidium violaceum, have been observed in Australia since 1984 [1]. An additional strain of the rust, F15, was widely released in temperate Australia as a biological control agent in the summers of 1991 and 1992 [2]. The impact of the rust on the biomass of several blackberry species has been monitored since 1984 in Victoria, with some species showing marked reductions in total biomass. There are, however, locations where rust disease is having minimal impact, even though the climate appears to be favourable for moderate to severe rust epidemics. We are therefore investigating the genetic interaction between strains of P. violaceum and various European blackberry taxa in Australia. Because the taxonomy of R. fruticosus agg. is problematic, we are identifying the genotypes of putatively named taxa. Our objective is to identify Rubus genotypes which are resistant to strains of P. violaceum present in Australia and thus determine whether or not additional strains of the rust should be introduced for the biological control of this weed.

Materials and methods
The biomass of blackberries was measured in winter at a number of sites in Victoria in the period 1984-88 and irregularly up to 1996. At each assessment, the following components of biomass were measured in 10 plots of 1x1 m: length of canes, number of floricanes, number of inflorescences per floricane, number of primocanes, number of daughter plants produced on each primocane, number of crowns per m2, total dry weight of roots and crowns per m2 and the total dry weight of canes per m2. In a related project, genotypes of Rubus were identified by DNA fingerprinting. Radiolabelled DNA of bacteriophage M13 was hybridized to total Rubus DNA digested with restriction-enzyme HaeIII. Rubus plants exhibiting restriction fragment length polymorphisms were considered to be different genotypes. The susceptibility of a Rubus genotype to a strain of P. violaceum was determined by inoculating detached leaflets or whole plants with urediniospores and, following an incubation period, assessing components of disease such as the latent period and the number of rust pustules per area of leaflet.

Results and conclusions
In the period 1984-88, total biomass of R. polyanthemus decreased 14% at Callignee, 37% at Beech Forest and 56% at Bonang. During the same period, R. procerus decreased by 23% at Myrtleford, R. ulmifolius sensu lato increased by 17% at Murrongowar, while during 1986-88 it decreased by 26% at Foster. Since 1988, these sites could not be monitored regularly, but the trends continue. Under suitable environmental conditions, disease caused by P. violaceum gradually reduces the biomass of blackberries and enables seed from other plant species to germinate and grow through the blackberry. In order to identify blackberries which are resistant to disease, we first identified 20 different Rubus genotypes among 13 putatively named Rubus taxa. Some taxa, such as R. ulmifolius sensu lato, are more genetically variable than others. We present preliminary results of studies to determine the susceptibility of Rubus genotypes to disease caused by strains of P. violaceum present in Australia. Strains of P. violaceum in Europe which are virulent on the genotypes of Rubus characterized as resistant in Australia could be identified and introduced to Australia.

References
1. Bruzzese E, Field RP, 1985. Proceedings 6th International Symposium Biological Control Weeds, pp. 609-612.
2. Bruzzese E, 1995. Proceedings 8th International Symposium Biological Control Weeds, pp. 297-300.