An increasing problem in the cultivation of arctic bramble (Rubus arcticus) is that annual yield fluctuates considerably and there is severe malformation and desiccation of developing fruits. Downy mildew (Peronospora rubi) has now been determined as the cause of these losses and the following evidence supports this conclusion. Purple-red interveinal necrosis occurred on leaves and microscopical studies showed intercellular mycelium and haustoria in affected tissues, with abundant conidiophores and conidia typical of P. rubi formed on the abaxial leaf surfaces. Sequence analysis of the ITS (internal transcribed spacer) regions of the ribosomal RNA gene repeat (rDNA) showed high homologies between downy mildew isolates from other Rubus and Rosa hosts and those from R. arcticus. Such ITS sequences will form the basis of species-specific PCR diagnostics for early testing of propagated material. The symptoms of the disorder were reduced and the yields increased by sprays of the fungicide metalaxyl compared with water-sprayed controls, but the elicitor -aminobutyric acid only delayed the onset of symptoms.