Identifying Oomycetes in a public garden

Oomycetes contain some of the most destructive plant pathogens, namely species from within the genera of Phytophthora and Pythium. Phytophthora ramorum is known to cause the mortality of a wide range of tree and woody shrub species, whereas Pythiums although more ubiquitous than Phytophthoras and commonly found in the same soil and waterbodies, are typically known to affect young plants and herbaceous species. The Royal Horticultural Society (RHS) is the UK's largest gardening charity, with five unique gardens in locations throughout England that house important tree and shrub, specimen plants as well as national collections. This research was undertaken in collaboration with the RHS after outbreaks of Phytophthora ramorum had been described at the RHS Rosemoor garden in Devon. The aim of this study was to identify Phytophthora and Pythium species within the garden. Twenty sites within the garden were included this study; soil samples were taken from around nineteen P.ramorum susceptible plant species and a sample of water was taken from the garden lake. The samples were then analysed to determine the presence of Phytophthora and Pythium species. Cultures were isolated from the soil samples using the apple baiting methods and genus identification was determined using morphological analysis of sporangia. Molecular identification of the samples to species level was confirmed by sequencing the internal transcribed spacer (ITS) of ribosomal DNA. A mixture of Phytophthora and Pythium species was found in seven of the soil samples. Phytophthora gonapodyides was identified in the water sample and the following Pythium species were confirmed soil samples taken from across the garden: P.attrantheridium, P.intermedium, P.macrosporum, P.senticosum, and P.sylvaticum. No correlation was found between plant species and pathogen species or pathogen species and garden location. It is interesting to know no Phytophthora species were identified in the soil samples analysed in this study, even though an outbreak of P.ramorum had been reported prior to sampling. Pythium senticosum was associated with a Cornus cultivar displaying disease symptoms. It is not known if this oomycete was the cause of these symptoms. To our knowledge, this is the first time P.senticosum has been reported in the UK. Further work is required to examine the pathogenicity of this species and potentially, this could be a novel pathogen in the UK.