The impact of Phytophthora disease on riparian populations of common alder(Alnus glutinosa) in southern Britain

In 1994 a survey was established to obtain information on Phytophthora dise ase of common alder (Alnus glutinosa) on the riverbanks of southern England and east Wales. Within an area of 70 000 km(2) 63 observation plots were s et up on stretches of river over 8 m wide. Average alder densities varied w idely in different parts of the survey area; from 0.7 to 22.2 trees per 100 m of river. From the density figures and data on the total length of rivers over 8 m wide within the survey area, it was estimated that there were app roximately 585 000 alder trees growing on the banks of such rivers. In 1994 3.9% of the trees showed crown symptoms of Phytophthora disease, an d an additional 1.2% of trees were dead, although not all of these had been killed ny Phytophthora. The disease occurred widely through the survey are a with an indication that the highest percentage of affected trees was to b e found in south-east of England. Subsequent surveys showed that the combin ed percentage of symptomatic and dead trees rose to 6.0% in 1995 and to 7.9 % in 1996. In the latter year, for an alder population of 585 000 trees, th is would correspond to 32 800 symptomatic and 13 500 dead trees. The percentage of trees showing symptoms was seven times as high in trees g rowing within Im of the riverbank as in trees growing between 1 and 10 m of the bank. An examination of the relationships between disease incidence an d various indices of water pollution revealed a positive association with t otal oxidized nitrogen. These results are discussed in relation to the biol ogy and possible origin of the disease.