First Report of Phytophthora citricola Occurring on Fagus sylvatica in Slovenia.

During 2005 and 2006, eight declining, mature Fagus sylvatica trees with severe crown dieback were encountered at two stands near Ljubljana and Kamnik in central Slovenia. Bleeding cankers were seen mostly in the lower parts of the stem. Fresh cankers showed orange brown necroses of the inner bark and cambium. Older cankers were dark brown and surrounded by cracks and shedding bark. Small pieces of marginal tissue were excised from fresh necrotic lesions and incubated in the dark at 20°C on P₅ARP and cornmeal (CMA) agar plates. Soil samples collected from around the base of these declining trees were submerged in sterile distilled water. Leaves of Rhododendron catawbiense were used as bait to test for the presence of Phytophthora spp. Several similar isolates were obtained from trees and soil from both stands. Colonies were narrowly petaloid, slow growing on P₅ARP, and fast growing on CMA. The colonies produced abundant sporangia when submerged in pond water. Semipapillate, noncaducous sporangia were ovoid to obpyriform, but were occasionally distorted, and 30 to 69 (49) μm long and 23 to 44 (34) μm wide. Neither hyphal swellings nor chlamydospores developed. Isolates were homothallic showing paragynous antheridia, spherical oogonia of 22 to 31 (25) μm diameter, and plerotic oospores of 20 to 27 (23) μm diameter. The morphological characters resembled those described for Phytophthora citricola (1). ITS rDNA, spanning ITS 1 and 2, plus the 5.8S rDNA were generated using primers ITS4 and ITS5. Isolates from symptomatic trees and the soil yielded identical sequences and were the same as various sequences deposited for P. citricola at GenBank. The ITS rDNA of one representative strain was deposited at GenBank (Accession No. EF423556). Mycelial plugs from one of the P. citricola strains grown on CMA were used to inoculate stem wounds of seven potted seedlings and wounds made on four freshly cut, healthy branches of F. sylvatica. Sterile agar plugs were used as controls. The test was carried out over 4 weeks at 20°C. Extensive necrotic lesions developed around inoculation points on seedlings and branches, whereas the controls showed no symptoms. P. citricola could also be reisolated from margins of these lesions. During the past decade, declining F. sylvatica trees were observed in an increasing number of stands in Germany (2). Several Phytophthora species appeared to be involved in this decline, but P. citricola was the most frequently recovered species (2,3). To our knowledge, this it the first report of P. citricola found associated with a decline of European beech in Slovenia. References: (1) D. C. Erwin and O. K. Ribeiro. Pages 282-287 in: Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (2) T. Jung. Forst Holz. 60:131, 2005. (3) T. Jung et al. Mycologist 19:159, 2005.