Occurrence of Phytophthora citricola in an alder forest in Hungary

During a survey to identify Phytophthora on alder, an isolate was collected in a swampy national reserve area near Budapest in June 2002. The pathogen was recovered from leaves of common cherry-laurel used to bait a soil sample containing necrotic roots of Alnus glutinosa (L.) Gaertner. The plantation did not show the severe disease symptoms characteristic of Phytophthora species hybrids associated with alder decline in many European countries including Hungary (Delcán & Brasier, 2001); ca. 5% of the trees showed slight crown dieback without bark necrosis on the collar or stem. Samples were taken around these trees. Pure cultures were obtained by placing the baits onto selective medium.

White, stellate colonies with little aerial mycelia were formed by one of the isolates on carrot, cornmeal, pea and potato dextrose agar media at 25° C (optimum temperature). Growth ceased between 30-32°C. Sporangia formed abundantly in non-sterile soil filtrate but not in agar. They were single, terminal or sometimes intercalary, non-caducous and semi-papillate; mostly obpyriform, rarely obovoid, limoniform or bifurcated. Exit pore was narrow. No hyphal swellings or chlamydospores were observed. The isolate was homothallic with rounded and smooth-walled oogonia, paragynous antheridia and aplerotic oospores. On the basis of morphological characters, our isolate was suspected to belong to Phytophthora citricola Sawada. This was confirmed by sequencing the ITS region of rDNA.

Pathogenicity tests were conducted by soil infestation on 2-year-old A. glutinosa seedlings. On average, 10% of the fine roots were necrotic 6 months after inoculation. The causal agent was consistently reisolated from these tissues, but the controls remained healthy. On stems and twigs, P. citricola initiated sunken necrotic lesions following wound-inoculations with mycelial agar plugs, similarly to the results of Brasier & Kirk (2001). However, differences in host resistance and/or pathogen aggressivity may exist, because another isolate did not infect the stem in similar test (Szabó, 2003). No other Phytophthora pathogenic on alder were collected in the sampled area.

P. citricola is often found in alder stands or in alder-lined rivers, and may be well adapted to attacking tree roots in wet environments (cf. Brasier & Kirk, 2001). Our data also suggest that it caused only root rot of alder in nature and that, in the lack of prominent above ground symptoms, the disease severity and progression are not very significant. This is the first report of the occurrence of a P. citricola isolate pathogenic on alder in Hungary.

This research was supported by grants of the Hungarian Scientific Research Fund (OTKA F 038325 and T 038309).