During surveys of forests in the Asian Region of Turkey in 1999, 2000, and 2001, mature trees of several oak species were observed to have symptoms of decline, including thinning of the crown, yellowing and wilting of leaves, dieback of branches, and growth of epicormic shoots on branches and the main trunk. Observations over time confirmed a slow progress of tree mortality. To isolate Phytophthora spp. that might be associated with the decline, samples of rhizosphere soil including fine roots with necrotic lesions, were collected from around the bases of individual trees. Young leaflets from Quercus robur, Q. petraea, and Q. hartwissiana were used in a bioassay to bait flooded soil subsamples. After 3 to 5 days, baits were transferred to plates of PARPNH, a medium selective for Phytophthora spp. (1). Phytophthora spp. were recovered from 38 of 51 sites investigated (75%). In all, 10 species of Phytophthora were isolated; P. quercina was detected most frequently and was obtained from 29 (57%) sampled sites. Identification of isolates of P. quercina was based on comparisons of cultural and morphological characteristics to the description of the holotype (2) and to authentic cultures. Slight differences from the description of the holotype were observed for the size of sporangia, oogonia, and chlamydospores. On V8 agar most of the colonies showed an optimal radial growth at 22.5°C, with a growth rate ranging from 4.5 to 5.6 mm per day. Isolates from Turkey showed a lower optimal temperature and a higher growth rate compared with European strains. Recent studies have shown that many Phytophthora species, including P. quercina, occur in declining oak stands in Europe. In pathogenicity tests of infested soil, P. quercina isolates proved to be one of the more pathogenic species to roots of young Q. robur plants (2). Previously, P. quercina was only found in Europe. New hosts species for P. quercina include Q. hartwissiana, Q. frainetto, and the endemic species Q. vulcanica. Observations suggest that P. quercina is widespread in Turkey and occurs within the natural range of oak, which raises the question of the role of P. quercina in the oak decline syndrome.
References: (1) T. Jung et al. Eur. J. For. Pathol. 26:253, 1996. (2) T. Jung et al. Mycol. Res. 103:785, 1999.
