Phytophthora ramorum and Phytophthora kernoviae in Scotland: status report

1. Background

History

P. ramorum was identified in 2000 as the pathogen responsible for the sudden death of oak trees in California and Oregon and for dieback of rhododendron nursery stock in The Netherlands and Germany, both of which had been observed since the early 1990s. It has since been found in the nursery trade and the wider environment in Europe. It was first found in GB in the nursery trade in 2002.

P. ramorum is regulated under EU emergency measures (2002/757/EC) to prevent its spread. This is implemented in national legislation through the Plant Health ( Phytophthora ramorum) (Scotland) Order 2004 and the Plant Health (Forestry) ( Phytophthora ramorum) (Great Britain) Order 2004.

In 2009, P. ramorum was found infecting Japanese larch ( Larix kaempferi) in south west England and outbreaks were found in Wales, Northern Ireland and the Republic of Ireland in 2010. In late 2010 a small infection was also detected on Japanese larch in Scotland. Information on outbreaks, biology and epidemiology of the pathogen and measures are available on the Forestry Commission web site (see section 8).

P. kernoviae was first discovered in Cornwall 2003 during inspections for P. ramorum (EPPO, 2013). P. kernoviae has also been identified in New Zealand and was found in Ireland in 2008. There has been very little spread outwith GB and it is rarely found in nursery stock. There is no specific EU legislation dealing with P. kernoviae, but in GB the same measures have been taken against it as are required for P. ramorum.

Biology

Both pathogens cause leaf blights and dieback on a wide range of shrub hosts and some trees, and bleeding bark cankers on certain tree hosts. They spread by producing spores which can be dispersed to other plants by water (rain-splash, irrigation, wind-blown rain or fog) or in soil or plant debris moved on footwear or vehicles. Movement of infected plants is a significant source of long-distance spread. Both pathogens can also persist for several years in soil and plant debris, infecting re-growth or replacement plants.

There are no currently available chemical treatments which can reliably kill P. ramorum or P. kernoviae, although some fungicides can reduce symptoms and sporulation. For this reason the use of fungicides is prohibited in nurseries where infection is found, to ensure that symptoms are not masked and infected plants can be identified. Eradication requires removal and secure disposal or burning of all infected plants, monitoring of regrowth and replacement with non-susceptible species. However, in established gardens, parks etc. the use of fungicides could be used to reduce inoculum or to protect valuable specimens.

The amount of spores produced varies between host plants. In most cases, spores are not produced on bark cankers, only on leaves. Rhododendron ponticum is one of the most significant sporulators in GB and until 2009 was a major source of spread, but generally limited to a few metres. Japanese larch, however, is an even more prolific sporulator of P. ramorum (and potentially at much greater height) as R. ponticum and evidence suggests it can sometimes be aerially dispersed up to 50 km or more.