Oomycete Diversity and Ecology in Declining Alder Stands in Switzerland

In this study, we assessed the occurrence and diversity of four oomycete genera (Phytophthora, Phytopythium, Pythium, and Globisporangium) in 13 declining alder (Alnus glutinosa and A. incana) stands in Switzerland. For this, we sampled and analyzed soil from tree rhizosphere, water from streams and rivers along which the stands were located, and symptomatic alder bark. The overall isolation rate was 47.2%, with a total of 400 oomycete isolates recovered at all 13 sites. The highest incidence of oomycete isolates was in soil samples (baiting, 82.5% isolation rate), followed by water (baiting, 14.7%), and bark (direct isolation, 2.7%). Of all recovered oomycete isolates, 90.3% could be successfully assigned to a known species, for a total of 23 species identified, including both preferential saprotrophs and pathogens. Among all genera, Phytophthora was the most abundant with 273 isolates (75.6%), followed by Phytopythium, Pythium, and Globisporangium. Oomycete species diversity showed a significant variation among substrates. Only one species-Phytophthora lacustris-was abundant in all substrates, while 16 species were restricted to a specific substrate, mainly soil. The rhizosphere of symptomatic alder trees harbored the most diverse oomycete community, highlighting once again the importance of soil as a reservoir for these microorganisms. Only two Phytophthora species were isolated from alder bark lesions, namely, P. × alni, the known causal agent of alder decline, and P. lacustris. The low recovery rate of P. × alni might be due to attempts to isolate it from old, inactive lesions, but may also suggest that alder decline might be caused by other oomycetes infecting the root system of the trees.