Pathogenicity of Phragmidium violaceum isolates on European blackberry clones and on leaves of different ages

Phragmidium violaceum (Pv) isolate F15 was released for biological control of invasive European blackberry in Australia in 1991-1992. This study reports variation in the pathogenicity of isolate F15 and Pv isolates SA1, V1 and V2, collected in southeastern Australia in 1997 or 1998, on a broad sample of genetically variable European blackberry clones collected in 1996-2000. The use of whole shoots or detached leaflet discs led to the same conclusions about the relative susceptibility of a blackberry clone to a particular Pv isolate. Moderate to relatively high numbers of uredinia developed on all 25 blackberry clones inoculated with isolate V1, whereas no or few uredinia of isolates V2 and SA1 developed on Rubus erythrops clone EB20, three clones of Rubus leucostachys (EB9, EB19 and 960804) or Rubus sp. clone SR43. Pv isolate F15 differed from the three isolates collected in Australia in that no or few uredinia of F15 developed on Rubus polyanthemus clone 961107 or three clones of Rubus laciniatus (EB22, KE1, SR14). Isolate F15 also differed from isolates V1 and V2 for the density of uredinia as a function of leaf age in a clone of Rubus anglocandicans found widely in Australia. Isolates V1 and V2 ceased production of uredinia on leaves older than 16days, whereas Pv isolate F15 continued production of uredinia beyond this leaf age. This study informed selection of additional Pv isolates from Europe for release in Australia in 2004.