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First Report of Leaf Spots on Laurus nobilis Caused by Phytophthora citricola in Belgium

Laurus nobilis (laurel tree) is used as an herbal and ornamental tree in gardens in Belgium. During the summer of 2001, a serious outbreak of leaf spots was observed in some Belgian nurseries. Symptoms were large, irregular, brown leaf spots located primarily on the distal half of leaves and delimited by a black margin. As the disease progressed, the spots enlarged, coalesced, and finally led to leaf withering. The isolated fungus had morphological characteristics typical of Phytophthora citricola. On potato dextrose agar (PDA) it formed white, entire rosette colonies. Within 1 week, semipapillate sporangia were found abundantly in water at room temperature. They were mostly ovoid and highly variable in size (20 to 82 × 12 to 48 µm). The sporangial apex was broadly papillate. Oogonia were spherical (18 to 38 µm in diameter), and the antheridia were paragynous. Pathogenicity of the isolated fungus was confirmed by inoculating two visibly healthy L. nobilis plants. Five leaves per plant were wounded by a scalpel, subsequently inoculated with 5-mm-diameter mycelial plugs on PDA, and sealed with Parafilm. As a control, noncolonized agar plugs were placed on wounded leaves from a third L. nobilis plant. The inoculated plants and the control plant were kept for 1 day under a plastic cover (approximately 95% relative humidity) on the laboratory bench. Within 1 week, all inoculated leaves developed symptoms, whereas the control leaves remained symptomless. P. citricola was sucessfully re-isolated, satisfying Koch's postulates. The pathogen was also characterized using molecular tools. The ribosomal DNA regions, ITS1 and 5.8S rDNA-ITS2, were sequenced and highest similarity scores were obtained with corresponding Phytophthora citricola sequence regions (>99% identity for both sequences). The new GenBank Accession Nos. are AY525786 (ITS1) and AY525787 (5.8S rDNA-ITS2). On the basis of the symptoms, cultural and morphological characteristics, and positive results in pathogenicity and PCR tests, the isolate was considered to be P. citricola. To our knowledge, this is the first record of P. citricola on leaves of L. nobilis in Belgium.