Life history of Phragmidium violaceum in relation to its effectiveness as a biological control agent of European blackberry

We report results of controlled-environment experiments conducted during 1982 and 1983 in Montpellier, France, in relation to the performance of Phragmidium violaceum as a biological control agent of the Rubus fruticosus aggregate (European blackberry) in Australia today. The phenology and duration of P. violaceum spore states appears to be similar in southern France and southern Victoria. Results of constant temperature experiments were consistent with weather conditions in Victoria associated with rapid disease increase; in particular, average maximum daily temperatures around 20°C and high relative humidity. Repeated inoculation of potted plants of R. anglocandicans reduced plant biomass equivalent to the effect of continually defoliating the shoot. A certain level of defoliation by P. violaceum each growing season appears to be necessary to reduce blackberry to an understorey species in Victoria. Sorus density declined exponentially with age of leaflets collected from shoots of potted plants of R. anglocandicans. We discuss the likely impact of age-related disease resistance on the efficacy of P. violaceum as a biocontrol agent and propose further modeling to predict times when shoot growth is synchronised with pathogen development. These results also have implications for the design of pathogenicity and host-specificity assays and indicate the potential for manipulating blackberry growth at key times to maximise disease severity.