Summary Dieback symptoms were observed on horse chestnut trees planted approximately 40 years ago in Ankara, Turkey. Lesions at the stem bases ofthe affected trees were similar to those of ink disease on sweet chestnut. A Phytophthora sp. was isolated from the fine roots and soil samplescollected around the stem bases by baiting using chestnut leaves. The pathogen was identified as Phytophthora citrophthora based on severalmorphological features and DNA sequences of the ITS region. Pathogenicity of P. citrophthora was tested by stem inoculation on 3-year-oldhorse chestnut saplings. P. citrophthora produced large cankers in 20 days and killed 40% of the saplings. This is the first report ofP. citrophthora causing dieback on horse chestnut. 1 Introduction Horse chestnut (Aesculus hippocastanum L.) is a native tree to the Balkans and is widely grown as an ornamental tree in theparks and along road sides in cities with a temperate climate (Walter 2006). It is commonly planted in the parks of Ankara,Turkey, but to date, no pathogens have been reported on this host in this country.Besides decay fungi, onlya few pathogens have been recorded on horse chestnut worldwide. Common diseases found on thishost are leaf blotch (Guignardia aesculi (Peck) V. B. Stewart) and anthracnose (Glomerella cingulata (Stoneman) Spauld. & H.Schrenk), while less important diseases are powdery mildew, caused by Erysiphe flexuosa (Peck) U. Braun & S. Takam, and leafspot, caused by Septoria hippocastani Berk. & Broome (Sabourin and Dykstra 2003). The most serious disease complexesaffecting A. hippocastanum in Europe, however, are pathogens causing bleeding canker including Phytophthora citricolaSawada, P. cactorum (Lebert and Cohn) J. Schro¨th and P. ramorum Werres et al. (Webber 2011). Recently, a new bacterialpathogen causing bleeding canker, Pseudomonas syringae pv. aesculi Durgapal and Singh, was reported in several Europeancountries, and it was added to the EPPO alert list (Schmidt et al. 2008; Webber et al. 2008).Phytophthora species have been reported causing bleeding canker of horse chestnuts in several countries in the world.Phytophthora cactorum is the most widely distributed species reported in Britain, USA, Czech Republic and Turkey (Brasierand Strouts 1976; Intini et al. 2002; Jung et al. 2005; Cerny et al. 2008). Apart from horse chestnut, this pathogen has alsobeen reported on beech (Fagus sylvatica L.) and white poplar (Populus alba L.) in the Czech Republic (Cerny et al. 2008).Jung and Burgess (2009) investigated Phytophthora species in forests, seminatural stands and nurseries in Europe andobtained homothallic Phytophthora isolates with paragynous antheridia, semipapillate persistent sporangia and a growthoptimum around 25 C; no catenulate hyphal swellings were formed on 39 host species in 16 families. These isolates differedgenetically from P. citricola, and therefore the name P. plurivora T. Jung and T.I. Burgess was proposed. The newly describedspecies was also isolated from aerial cankers, rotten collar, rhizosphere soils and nurseries of horse chestnut. Phytophthoracitricola sensu lato had been isolated from horse chestnut, on which it was shown to be pathogenic previously (Brasier andStrouts 1976). Jung et al. (2009) also found P. cambivora (Petri) Buisman on horse chestnut.Werres et al. (1995) isolated P. cactorum, P. citricola (sensu lato) and P. syringae (Berk.) Kleb. from the soil around diseasedand dying horse chestnut treesat three different sites in Germany. The infected stems had rotten bark with a gummy exudation.Only P. cactorum could be isolated from diseased and dead stem tissues. Infection trials with 3-year-old A. hippocastanumseedlings showed P. cactorum and P. citricola isolates to be pathogenic, whereas symptoms could not easilybe reproducedwithP. syringae. More recently, Brown et al. (2005) reported P. ramorum as causal agent of bleeding lesions on horse chestnut.Phytophthora citrophthora (R. E. Smith and E. H. Smith) Leonian is a polyphagous and damaging pathogen causing foot androot rot worldwide. Erwin and Ribeiro (2005) listed 82 hosts for this pathogen, including many forest trees and some annualplants, but excluding horse chestnut. This pathogen is mostly present in warmer regions, but it does not produce oospores andchlamydospores, and does not survive at temperatures over 33 C.Alvarez et al. (2011) reported considerable losses in recent years in Spain because of Phytophthora branch canker ofclementines caused by P. citrophthora. A total of 134 isolates of P. citrophthora were collected from 2003 to 2005 in 135 citrusorchards in Spain, and 22 reference isolates were analysed genotypically and phenotypically to determine the populationstructure. Phytophthora branch canker of clementines was associated mainly with a single genotype (P-1), which included 88%of isolates obtained from branches. Strains isolated years before the first disease outbreak also clustered with this majorgenotype, which was considered as a predominant population. Although great variation in the morphological and physiologicalFor. Path. 42 (2012) 299–304 doi: 10.1111/j.1439-0329.2011.00754.x 2011 Blackwell Verlag GmbH