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

Abstract

In 2007, declining mature trees of common beech (Fagus sylvatica), horse chestnut (Aesculus hippocastanum), and white poplar (Populus alba) with bleeding cankers were found in several locations in the Central Bohemia region. The morphological and cultural characteristics of isolates from diseased tissues were consistent with those of Phytophthora cactorum (Erwin & Ribeiro, 1996). The colonies were stellate to rosaceous, appressed with slight aerial mycelium on V8 agar. Radial growth was 8–14 mm day−1 at 20°C. Optimum growth temperature was 23–25°C, minimum 3–8°C and maximum 30–33°C. Strains were homothallic, produced abundant terminal, smooth-walled, spherical oogonia (20–35 µm in diameter). Oospores were plerotic or nearly plerotic, 22–31 µm in diameter and the oospore wall 1·0–2·2 µm thick. Antheridia were paragynous, but amphigynous antheridia also occurred in up to a quarter of cases. Sporangiophores were sympodial, sporangia ellipsoidal, ovoid to subglobose, papillate and measured 17–38 × 14–30 µm (L:B ratio 1·05–1·80). Comparison of DNA sequences of ITS region of representative isolates from all hosts (GenBank Acc. Nos. EU562207–EU562209) gave 99% identity to P. cactorum. Isolates 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 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 2 to 3 months and in c. 33% of poplars, with the majority of poplar saplings forming callus and surviving the infection. The pathogen was successfully reisolated from plants showing symptoms. The control plants inoculated in the same way with sterile agar plugs remained healthy. Phytophthora cactorum is well known pathogen of beech and horse chestnut (Brasier & Strouts, 1976; Jung et al., 2005), but not reported 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 the first record of it on white poplar. The work was supported by project no. QH71273 of the Ministry of Agriculture of the Czech Republic.