The potential of symptomless potted plants for carrying invasive soilborne plant pathogens

The most common pathway for the movement of plant pests across borders is the horticultural trade in live plants, especially potted ornamentals. Soilborne pathogens, possibly alien and potentially invasive, have a higher possibility of surviving transportation and becoming established at their destination if they are carried in potted plants. The European Union (EU) has an open-door phytosanitary system, under which any plant that is not specifically regulated can be imported. Inspections are focussed on a small number of economically important plant pests and even then limited to visual examinations of the aerial parts of the plant. Inspections fail to detect regulated pests or others internal to the tissues, or in the soil, if plants appear asymptomatic, or if incipient symptoms are limited to the roots. Phytophthora, a soilborne pathogen universally infamous for its ruinous outbreaks, but poorly regulated in Europe, was chosen to illustrate the risk inherent in the nursery pathway. The aim of this study was to demonstrate the level of infestation by Phytophthora in ornamental plants largely traded to, from and within Europe. Location European Union. Methods As Phytophthora species are not easily isolated, a real-time PCR assay was developed, based on a genus-specific TaqMan MGB probe, to detect the pathogens in plant tissues or soil even when present at low concentrations, and before symptoms occurred. Phytophthora species were identified by isolation and sequencing of the ITS (internal transcribed spacer) region. Results Phytophthora was detected by qPCR in 87% of the tested pots and in 70% of the asymptomatic potted plants. Potted plants in soil carried several Phytophthora species without showing any external symptoms. Main conclusions The results of this study strongly support the case for more rigorous European legislation on the trade of live plants in pots. As eradication of soilborne organisms is difficult, if not impossible, an embargo on plant movements into the EU and between member states is the only advisable measure against the spread of these pathogen