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Distribution of the Phytophthora disease of alder
As a result of investigations conducted by Forest Research over the past 14 years, 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. In the UKOn 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:
- Austria
- Belgium
- Czech Republic
- France
- Germany
- Hungary
- Ireland
- Italy
- Lithuania
- The Netherlands
- Poland
- Slovakia
- Slovenia
- Sweden.
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 a similar pathogen may have been found in a nursery in Minnesota, USA, infecting a woody ornamental species. 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:  Another new? species of Phytophthora on alder down under (PDF-2008K)
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