Downy mildew (Peronospora rubi) in Rubus arcticus: A threat to commercial derry production in Finland

An increasing problem in the cultivation of arctic bramble (Rubus arcticus) is that annual yield fluctuates considerably and there is severe malformat ion and desiccation of developing fruits. Downy mildew (Peronospora rubi) h as now been determined as the cause of these losses and the following evide nce supports this conclusion. Purple-red interveinal necrosis occurred on l eaves 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 (in ternal transcribed spacer) regions of the ribosomal RNA gene repeat (rDNA) showed high homologies between downy mildew isolates from other Rubus and R osa hosts and those from R. arcticus. Such ITS sequences will form the basi s of species-specific PCR diagnostics for early testing of propagated mater ial. 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 P-aminobutyric acid only delayed the onset of symptoms.