Plants derived from micropropagated stocks of several blackberry and blackberry x red raspberry cultivars have become infected with Peronospora rubi Rabenh. in the United Kingdom in the last 3 years. In inoculation tests on detached leaves from plants grown in vitro, P. rubi infected red raspberry, blackberry and hybrid berry genotypes. The fungus developed an extensive intercellular systemic mycelium and conidia before necrotic lesions were visible: it then produced oospores as some necrosis appeared.