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

Bleeding canker and decline of riparian alder populations has been animportant problem in many European countries, including the CzechRepublic. Initially, an ‘alder Phytophthora’ was isolated from damagedblack alder trees in western Bohemia near the town of Karlovy Vary in2001 (derny et al., 2003). The exact species identiﬁcation was unknownat this time. Since its original discovery, we have isolated similar Phytoph-thora species from damaged innerbark and conductive tissues of decliningtrees of Alnus glutinosa and A. incana from approximately 60 alder standsin the Czech Republic. The pathogen has been spreading rapidly in theaffected alder stands, particularly in the western part of the Czech Republic.The pathogen has been frequently found in catchments of the Vltava riverand Ohre river (western, northern, middle, southern Bohemia, westernpart of Vysocina region). Extensive decline of alder trees has not beendetected in the eastern part of country (Moravia) yet, but the pathogen hasbeen 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 causalagent of alder decline in Britain was determined to be a new hybrid,designated as Phytophthora alni subsp. alni (Brasier et al., 2004). Symptoms on diseased trees were characteristic of bleeding canker andalder decline in other regions (Jung & Blaschke, 2004): small, yellowing,and sparse foliage, dieback in the canopy and bleeding cankers on treetrunks. Many isolated cultures were similar morphologically and hadcharacteristics that were consistent with P. alni subsp. alni (Brasier et al.,2004). Colonies growing on carrot agar (CA) were uniform, appressedwith sparse aerial mycelium. Radial growth was 7–9 mm per day at 20 °Con CA. Optimal growth temperature was 23–25°C , with several isolatesfailing to grow at 6 and 33°C. Isolates were homothallic with two-celled(22–31 × 12–20 μ m) amphigynous antheridia, producing abundantterminal, spherical oogonia (28–55 μ m in diam.) with moderatelyornamented walls. Many oogonia aborted at a rate of 20 to 60%. Sporan-giophores were simple with terminal sporangia, proliferating internally,often nested, ellipsoid in shape, measuring 38–65 × 25–41 μ m (length × width ratio 1·4–1·6), and had minute papilla or were non-papillate.Comparison of DNA sequences of ITS region of two isolates (GenBankAcc. Nos EF194776, EF194777) with acquired sequences with thosedeposited in GenBank, conﬁrmed their identity as P. alni. Pathogenicitywas tested by artiﬁcial infection of 4-year-old black alder plants. Twentyplants were inoculated by placing segments of agar with mycelium, takenfrom the colony margin growing on CA, on the underbark at the collar ofplants. The characteristic necroses of collars and wilting of plants wereobserved after several weeks and the pathogen was reisolated from damagedtissues. The control group of alder plants inoculated with sterile agarremained healthy. This is the ﬁrst positive identiﬁcation of P. alni with proofof its pathogenicity of black and grey alder reported in the Czech Republic.ReferencesBrasier CM, Kirk SA, Delcan J, Cooke DEL , Jung T, Man in’t Veld WA, 2004.