As a result of investigations conducted by Forest Research over the past two decades, it has become clear that in the UK Phytophthora disease of alder is now widespread in the riparian ecosystems where alder commonly grows.
The principal information on the scale of the disease comes from a series of riparian plots established in 1994 in the southern half of England and east Wales on rivers over 8 m wide. The percentage of trees affected by Phytophthora alni has increased steadily each year. By 2003 the survey indicated that around 16% of the total was diseased or dead. It is now estimated that 20% of trees in this area are affected.
In the UK
On average, the disease incidence is highest is southeast England. However, heavy losses are occurring in some of the large alder populations that occur along western rivers – for example, in the Marches and parts of Wales.
The disease is also causing damage to alders on Scottish river systems, including on the Rivers Avon, Dee, Deveron, Duirinish and Spey.
In Europe and elsewhere
Apart from the UK, alder disease is now widespread in Europe. It has been reported from:
- Czech Republic
- The Netherlands
Very high losses have occurred in some localities (such as northeastern France and Bavaria in Germany); in others the disease impact is not yet so serious.
Until 2006 the alder Phytopthora had not been found outside Europe, but a recent report suggests that it was found in a nursery in Minnesota, USA, infecting a woody ornamental species.
More recently, P. alni subsp uniformis (PAU) has been discovered in Alaska associated with native alders but in the absence of any damage. The distribution pattern of the PAU positive sites across over 1,000 miles of road suggests it may be a native pathogen. Testing of these isolates and comparisons with PAU isolates from Europe is underway to determine whether the organism is native or introduced in Alaska.
A different species of Phytophthora, yet to be identified but apparently different from P. alni, has also been found causing a similar root and collar disease of alder in Australia: