Detection of oomycete pathogens in UK peat-free growing media and implications for plant health

Peat-based growing media products are being phased out in the UK. Alternative constituents may be at risk of harbouring oomycete plant pathogens. Thirty-five peat-free growing media samples were tested for oomycete pathogens by baiting and DNA metabarcoding. No Phytophthora species was detected by baiting, but seven species of Pythium, Phytopythium and Elongisporangium, including four species not previously reported in the UK, were baited. DNA matching eleven Phytophthora species was detected by metabarcoding across a range of samples. A coir sample was also found to contain DNA of Peronosclerospora, a genus of tropical downy mildew pathogens. DNA matching six Phytophthora pathogens, including P. bishii, a pathogen of strawberry not previously reported in the UK, was detected in recycled coir that had been used for strawberry production in The Netherlands. We discuss the implications of the species detected in terms of risks to plant health and the need for further evidence-gathering.