First record of the downy mildew Peronospora myosotidis on Brunnera macrophylla in the UK

In September 2004, a sample of Brunnera macrophylla cv Jack Frost (Boraginacae) with numerous discrete, angular, brown necrotic leaf spots, was submitted to the Central Science Laboratory for diagnosis. The plants had been raised in southern England by micropropagation and grown in a well ventilated polytunnel with sand capillary irrigated and drained beds. No recent import connection was found. About 60% of the plants in the polytunnel were showing symptoms that were initially observed three weeks earlier. On the lower surface of the variegated pubescent leaves, sporulation typical of a downy mildew was associated with the leaf spots. Sporangiophores were dichotomously branched 5 to 8 times, and sporangia were ellipsoid to globose, thin-walled and light-brown in colour (18-22 x 21-24 µm). The morphology of the fungus was typical for Peronospora myosotidis (Gaumann, 1923). The only previous record of a downy mildew on B. macrophylla is Peronospora anchusae on Siberian bugloss (as Anchusa myosotidiflorae) in USSR from 1927 (Farr et al., n.d.). There are no previous records of a downy mildew on B. macrophylla in the British Isles, but there are records of P. myosotidis on other members of the borage family (Francis & Waterhouse, 1988). The pathogen was first described in 1863 on leaves of several members of the Boraginaceae in parts of Europe and North America (Saccardo, 1888) and is now reported on eight different genera from across the world, affecting both leaves and stems. B. macrophylla (also known as false-forget-me-not) is grown commercially due to its attractive blue flowers and heart shaped leaves, providing excellent ground cover in shaded areas. This is the first record of Peronospora myosotidis on Brunnera macrophylla in the UK and it is believed to be due to a natural extension of the pathogen's host range.