The role and diversity of Pythium and Phytophthora in UK gardens

Abstract

Gardens are little studied particularly in relation to major plant pathogen genera such as Phytophthora, or the closely related Pythium. UK gardens harbour a wide diversity of plants of worldwide origin, compared to the relatively few native in the UK, and are frequently the endpoint of the worldwide trade in plants and sometimes, as fellow passengers their associated pathogens.

Samples from a plant clinic were surveyed for the presence of Phytophthora by three methods. DNA extracted from symptomatic tissue followed by a semi-nested PCR (DEN) gave the highest detection rates with approx. 70% of tests positive. A commercial immunoassay test kit (PocketDiagnositic™) was the fastest; with results in less than 10 min. Apple baiting gave the lowest detection rates (9%), but provided cultures vital for further studies. An unexpected and novel result was the widespread detection of Pythium causing much the same symptoms as Phytophthora.

The phylogenetic trees, created using the elision method, of the Phytophthora and Pythium rDNA sequences revealed 46 named or well defined species, 21 and 25 respectively. The phylogeny of both genera was in general accordance with previous publications. Frequently identified species included Ph. cryptogea, Ph. cinnamomi, Py. intermedium and Py. sylvaticum, all ubiquitous with wide host ranges. Occasional occurrences included Ph. ramorum, Ph. tropicalis, Ph. austrocedri and Ph. “niederhauseri”. Twenty putative new species were also detected, based on the Phytophthora and Pythium phylogenies, 11 and 9 species respectively.

In pathogenicity tests Phytophthora and Pythium caused root rot, and Py. intermedium caused Hebe death within 3 days of soil inoculation. Not all plants infected with Phytophthora or Pythium died, indicating disease development may involve additional interactions. Pythium foliar blight of mature woody plants was identified. Koch’s postulates were satisfied, indicating new symptoms previously not associated with Pythium infections.