Phytophthora alni causing decline of black and gray alders in the Czech Republic

Bleeding canker and decline of riparian alder populations has been an important problem in many European countries, including the Czech Republic. Initially, an “alder Phytophthora“ was isolated from damaged black alder trees in western Bohemia near the town of Karlovy Vary in 2001 (Černý et al., 2003). The exact species identification was unknown at this time. Since its original discovery, we have isolated similar Phytophthora species from damaged innerbark and conductive tissues of declining trees of Alnus glutinosa and A. incana from about 60 alder stands in the Czech Republic. The pathogen has been spreading rapidly in the affected alder stands, particularly in the western part of the Czech Republic. The pathogen has been frequently found in catchments of the Vltava river and Ohre river (western, northern, middle, southern Bohemia, western part of Vysocina region). Extensive decline of alder trees has not been detected in the eastern part of country (Moravia) yet, but the pathogen has been found in watercourses in the upper part of catchment of the Dyje river (Vysocina region) and is likely spreading to the east. In 2004, the causal agent of alder decline in Britian was determined to be a new hybrid, designated as Phytophthora alni subsp. alini (Brasier et al., 2004).

Symptoms on diseased trees were characteristic of bleeding canker and alder decline in other regions (Jung & Blaschke, 2004): small, yellowing, and sparse foliage, dieback in the canopy and bleeding cankers on tree trunks. Many isolated strains were similar morphologically and had characteristics that were consistent with P. alni subsp. alni (Brasier et al., 2004). Colonies growing on carrot agar (CA) were uniform, appressed with sparse aerial mycelium. Radial growth is 7-9 mm per day at 20 °C on CA. Optimal growth temperature was 23-25 °C , with several strains failing to grow at 6 and 33 °C. Strains were homothallic with two-celled (22-31 x 12-20 μm) amphigynous antheridia, producing abundant terminal, spherical oogonia (28-55 μm in diam.) with moderately ornamented walls. Many oogonia aborted at a rate of 20 to 60 %. Sporangiophores were simple with terminal sporangia, proliferating internally, often nested, ellipsoid in shape, measuring 38-65 x 25-41 μm (length x width ratio 1.4-1.6), and had minute papilla or were non-papillate. Comparison of DNA sequences of ITS region of two isolated strains (GenBank accession numbers EF194776, EF194777) with acquired sequences with those deposited in GenBank, confirmed their identity as P. alni. Pathogenicity was tested by artificial infection of 4-years old black alder plants. Twenty black alder plants were inoculated by placing segments of agar, taken from the colony margin growing on CA, on the underbark at the collar of plants. The characteristic necroses of collars and wilting of plants were observed after several weeks and the pathogen was reisolated from damaged tissues. The control group of alder plants inoculated with sterile agar segment remained healthy. This is the first positive identification of P.alni with proof of its pathogenicity of black and gray alder reported in the Czech Republic.