Boxwood (Buxus spp.) includes several species of popular ornamental shrubs used in traditional and contemporary gardening. In March of 2008, a gradual and irreversible decline was observed on 6-year-old Buxus rotundifolia plants in a garden in central Italy. Of 150 boxwood plants, 70% were symptomatic, and among them, approximately 25% were completely wilted. Aboveground symptoms included stunting, necrotic bark lesions at the base of the stem, reduced growth, and leaf chlorosis. Leaves at first appeared light green, then turned yellow, bronze, or straw colored. Foliar symptoms were restricted to a few branches or extended to the whole crown. Immunological field tests (Pocket Diagnostic, CSL Diagnostics, Milan, Italy) on necrotic rootlets and bark tissues suggested the risk of the presence of Phytophthora spp. One species was consistently isolated on PARHP (2) from necrotic tissues of three symptomatic plants. Recovered isolates were heterothallic and produced aerial mycelium. Star-like compact growth was also observed on potato dextrose agar. Cultures on carrot agar submerged in saline solution (2) developed papillate, noncaducous, and sympodially branched sporangia of different shapes ranging from ovoid to obturbinate. Sporangia were 37.5 to 65.0 × 22.5 to 35.0 μm (average 49.0 × 31.0 μm); L/W ratio from 1.1 to 2.1 (average 1.6). On the basis of morphological features, these isolates belong to P. citrophthora (R.E. Sm. & E.H. Sm.) Leonian (2). The identity was confirmed by sequencing of the internal transcribed spacer region of the rDNA (NCBI Accession No. FJ874794). Inoculum for two isolates of P. citrophthora (3BO and 4BO) was produced for pathogenicity testing on 100 g of autoclaved millet grains moistened with 70 ml of V8 juice. Two, 2-liter aliquots of potting media were each amended with the infested grains (1:20 vol/vol) and five 3-year-old plants of B. rotundifolia per isolate were transplanted into the media. Plants were maintained in a growth chamber at 18 to 22°C, with relative humidity of 80%, and a 12-h photoperiod. Sterile millet grains were used to inoculate five boxwood plants for the control. After 2 months, control plants remained healthy while inoculated plants displayed root necroses and foliar symptoms similar to those observed in the field. The pathogen, consistently reisolated from roots of infected plants, reduced the root weight by an average of 25%. Root rot of boxwood plants is reported to be caused by P. citricola in Poland (4), P. parasitica in Virginia (3), and P. citrophthora in North Carolina (1). To our knowledge, this is the first report of P. citrophthora on B. rotundifolia in Europe.
References: (1) Z. G. Abad et al. Plant Dis. 78:830, 1994. (2) D. C. Erwin and O. K. Ribeiro. Page 562 in: Phytophthora Diseases Worldwide. The American Phytopathological Society. St. Paul, MN, 1996. (3) M. A. Hansen. Plant Dis. Fact Sheets. 450-614W, 2000. (4) L. B. Orlikowski. J. Plant Prot. Res. 46:163, 2006.
