Phytophthora cactorum causing bleeding canker of common beech, horse chestnut, and white poplar in the Czech Republic

In 2007, declining mature trees common beech (Fagus sylvatica), horse chestnut (Aesculus hippocastanum), and white poplar (Populus alba) with bleeding cankers were found in several locations in Central Bohemia region. We acquired many similar Phytophthora isolates from the diseased tissues. Their morphological and cultural characteristics were consistent with those of P. cactorum (Erwin & Ribeiro, 1996). The colonies were stellate to rosaceous, adpressed with slight aerial mycelium on V8 agar. Radial growth was 8-14 mm/day at 20 °C. Optimum growth temperature was 23-25 °C, minimum 6-8 °C and maximum 30-33 °C. Strains were homothallic, produced abundant terminal, smooth-walled, spherical oogonia (20-35 μm in diameter), oospores plerotic or nearly plerotic, 22-31 μm in diameter, oospore wall 1.0-2.2 μm thick. Antheridia were paragynous, but amphigynous antheridia occurred at any rate (up to 25%). Sporangiophores were sympodial, sporangia ellipsoidal, ovoid to subglobose, papillate, measured 17-38 × 14-30 μm (L:B ratio 1.05-1.80). Comparison of DNA sequences of ITS region of representative strains from all hosts (GenBank Acc. Nos. EU562207 - EU562209) confirmed the 99 % identity to P. cactorum. Strains were deposited at Culture Collection of Fungi, Prague under CCF Nos. 3757, 3758, 3762.

The pathogenicity was tested using 20 1-year-old saplings of all three hosts. Wounds (5 mm diam.) on stems of ten saplings of each host were artificially inoculated by plugs of V8 agar from the actively growing colony margins of corresponding isolates and the wounds were sealed by Parafilm. The characteristic stem necroses developed after 1-2 weeks on all plants. Stems of many beech and horse chestnut saplings were girdled in two to three months but about 33% of poplars as well, with most poplar saplings forming callus and surviving the infection. The pathogen was successfully reisolated. The control plants inoculated in the same way with sterile agar plugs remained healthy.

P. cactorum is well known pathogen of beech and horse chestnut (Brasier & Strouts, 1976; Jung et al., 2005), but we have not found any information about incidence of P. cactorum on any poplar species. This is the first report of P. cactorum causing bleeding canker of common beech and horse chestnut in the Czech Republic and probably the first report of it on white poplar in the world.