The cardinal temp. for in vitro germination of conidia of imported and indigenous isolates of downy mildew from hosts in the genera Rubus and Rosa were similar. A high percentage of conidia germinated at >2C and germination remained between 80 and 90% up to 15C or 20C, depending on the isolate. The highest incidence of disease on leaf discs of Tummelberry (blackberry x red raspberry) inoculated with an isolate of Peronospora rubi occurred at c. 15C, with infection over a range from 2C-28C. Tests on leaf discs in vitro and leaflets of primocane and lateral shoots in plastic tunnels, with 3 hybrid berry (blackberry x red raspberry), 6 blackberry and 9 red raspberry cultivars showed the hybrid berries to be most susceptible. In a plastic tunnel infected drupelets of red raspberry fruits developed more slowly and failed to ripen evenly compared with uninfected drupelets. Similar malformation of infected fruits occurred in a plantation of Tummelberry. An isolate of P. rubi attacked both Tummelberry and rose cv. Can Can. Fluorescence microscopy after staining with aniline blue showed that leaf discs of Tummelberry were extensively colonized by intercellular mycelium of P. sparsa isolated from rose, even though sporulation was sparse or absent. It is suggested that P. rubi and P. sparsa may be conspecific. Oospores of P. rubi were found routinely within leaf discs of Rubus cultivars inoculated in vitro and once in naturally infected leaflets of Tummelberry.