First report of Septoria betulae causing leaf spot of birch in the United Kingdom

In August and September 2004, a survey was conducted to evaluate diseases of birch (Betula spp.) in Scotland, since it is a major broadleaved component of recent, upland, native woodland planting. At three sites in south-west Scotland, a number of downy birch (Betula pubescens) trees were found to have unusual leaf spots. The spots were small (up to 3-4 mm in diameter), although frequently coalescing, circular-to-irregular and ochre-to-pale brown, with darker margins (Fig. 1).

On the surface of these spots were numerous black pycnidia, which produced hyaline conidia, 4-7 septate, generally filiform although tapering at one end, measuring 45-75 x 2-3 µm (Fig. 2). Single conidial isolations onto 2% malt agar consistently formed slow growing (1.5 cm diameter after 20 days), dark green colonies (Fig. 3). Pycnidia matured after 3-4 weeks when plates were incubated at 15-20°C with a 12 h photoperiod consisting of cool white fluorescent and near-ultra violet light. Conidia were morphologically similar to those produced on infected leaves. An isolate of this fungus was formally identified as Septoria betulae by the Centraalbureau Voor Schimmelcultures (CBS), Utrecht, The Netherlands, and deposited in the CBS culture collection.

To confirm the pathogenicity of the isolate of Septoria betulae, a suspension of 3 x 10⁵ conidia per ml in water was applied to seedlings of downy birch, at a rate of 4 ml per seedling. Inoculated seedlings were incubated at 20/15°C day/night temperatures, with a 16 h photoperiod and misted for the first 6 days. Leaf spots similar to those observed on naturally infected trees developed on inoculated seedlings within 14 days; whereas control seedlings remained healthy. Septoria betulae was re-isolated from leaf spots on inoculated seedlings, thus satisfying Koch's postulates.

Septoria betulae has been recorded on several birch species in Eurasia and North America (Constantinescu, 1984) but not in the UK (Bennell & Millar, 1984) and is absent from current literature detailing the fungal associations of birch in this country. Therefore, this is the first confirmed record of Septoria betulae infecting birch in the UK, although its significance as a pathogen has yet to be determined.