Publication Type:Journal Article
Source:Plant Disease, Volume 100, Issue 5, p.1024 (2016)
Phytophthora pseudosyringae causes stem necrosis, and root and collar rot of several woody tree species (Fagus sylvatica, Alnus glutinosa, Castanea sativa, Nothofagus spp., and Quercus spp.) across Europe (Jung et al. 2003; Pintos Varela et al. 2007; Scanu and Webber 2015) and severe dieback of heathland plants such as bilberry (Vaccinium myrtillus) in the UK (Beales et al. 2010). In June 2014, a symptomatic horse chestnut tree (Aesculus hippocastanum) was detected in Sankt Jörgens Park in Gothenburg, southern Sweden (57°45′1.5″ N; 11°57′8.1″ E; elevation 22 m). The tree displayed 30% crown dieback and several tongue -shaped necrosis at the base of the stem. The outer bark over the bleeding patches was removed and small pieces of infected phloem from the margin of the necrosis were directly plated onto CMA-PARPBH selective medium (17 g/liter corn meal agar (CMA), 0.125 g/liter sodium ampicillin, 0.02 g/liter benomyl, 0.07 g/liter hymexazol, 0.1 g/liter PCNB, 0.01 g/liter primaricin, and 0.01 g/liter rifamycin) and incubated at 20°C. Growing hyphae were later transferred to V8 agar and incubated at 20°C in darkness. After 7 days, the colony reached 5.4 cm in diameter and displayed a stellate growth pattern with faint aerial mycelium. Sporangia were sympodial, semipapillate, mostly ovoid, less commonly limoniform to ellipsoid, with a length/breadth average ratio of 1.44 μm (SE = 0.03). Oogonia were 28.1 μm (SE = 0.46) in diameter, smoothly walled, spherical, and with paraginous antheridia, although some amphiginous antheridia were observed. Morphological features and growth pattern on V8 agar corresponded to those of P. pseudosyringae. DNA was extracted from mycelia and the ITS region was amplified and sequenced using the ITS4 and ITS6f primers (Cooke et al. 2000) and deposited in GenBank (Accession No. KU257470). BLAST search showed 99 to 100% identity with reference sequences of P. pseudosyringae deposited in GenBank. Under-bark inoculation with mycelium plugs was performed on 10 one-year-old excised shoots obtained from three different horse chestnut trees. Inoculated shoots were placed over autoclaved and moistened filter paper inside sterile Petri dishes and incubated at 23°C in darkness. After 14 days, the length of the lesion on inoculated shoots was significantly higher than on control shoots (8.53 mm, SE = 0.74 vs. 0.91 mm, SE = 0.93; P < 0.0001). The pathogen was reisolated from all infected shoots fulfilling Koch postulates. To our knowledge, this is the first report of P. pseudosyringae in Sweden and the first report of P. pseudosyringae causing basal cankers and dieback on horse chestnut worldwide. The number of recent reports of damage caused by P. pseudosyringae raises concerns about the potential impact of this pathogen on horse chestnut trees, widely planted as ornamentals in Sweden, as well as to natural beech and oak forests across the country.