Risk factors for the Phytophthora-induced decline of alder in northeastern France

A lethal disease of common alder caused by Phytophthora alni, a new hybrid pathogen, has been spreading in Europe since the early 1990s. In 2004, we conducted an epidemiological survey in northeastern France to determine disease frequency and to investigate the impact of environmental factors on disease prevalence. Seventy-eight plots in the Rhin- Meuse basin were investigated. The survey was structured to enable critical examination of the possible impact of nitrogen pollution of the river water on disease prevalence. P. alni-induced alder decline was common throughout northeastern France. Altogether, disease was found in 80% of the plots containing alder, with 16% of all the alders affected. Striking differences existed between watercourse types. Lower proportions of diseased alders were found in watercourse types with rapid water flow, such as mountain streams of the Vosges and piedmont or watercourses on steep calcareous slopes, than in the slow watercourses of the low-lying valleys of the calcareous plateaus and of the clayey plains. Disease prevalence was not related to the total oxidized nitrogen concentration of the water. However, prevalence increased with the mean summer temperature of the river water and where clayey soils were found in the river banks. The results of this work can be used for the assessment of P. alni-induced alder decline risks in affected European countries and in areas where the disease could be introduced.