First report of Phytophthora nicotianae as pathogen of blue Mediterranean fan palm

Blue mediterranean fan palm (Chamaerops humilis var. argentea) is a shrub-like palm with clumps that are pale silvery-blue in colour. This species is native to the western Mediterranean basin and appreciated as an ornamental in Europe. During the summer of 2009, 40% of a nursery stock of approximately 30,000 three-year-old potted blue Mediterranean fan palms growing in the open air, in an ornamental nursery in eastern Sicily (Italy), showed wilting and dieback. Initial symptoms were a dark brown rot on the petiole base and blight of the unopened spear leaves (Fig. 1). Foliar symptoms were associated with the browning of basal stem and root rot (Fig. 2). A Phytophthora species was consistently recovered by plating rotted tissues of plants showing symptoms onto a selective medium (Masago et al., 1977), and pure cultures were obtained by single-hypha transfers.

The species was identified by morphological and cultural characters (Erwin & Ribeiro, 1996) as Phytophthora nicotianae Breda de Haan. Isolates formed stoloniferous colonies on potato dextrose agar (PDA) and grew between 8°C and 36°C, with the optimum at 30°C. On V8 juice agar, they produced persistent, mono- and bipapillate, spherical to ovoid, ellipsoid, obpyriform sporangia (28-54 x 42-46 µm; length/breadth ratio of 1.3:1). All isolates were A1 mating type and formed spherical oogonia with smooth walls (mean diameter 26 ± 2 µm) and amphigynous antheridia in dual cultures with reference P. nicotianae isolates of A2 mating type. Spherical, intercalary chlamydospores were produced. The internal transcribed spacer (ITS) region of rDNA of a representative isolate (IMI 398853) from C. humilis var. argentea was amplified using primers ITS6/ITS4 (Cooke et al., 2000), sequenced and deposited in GenBank (Accession No. HQ287571). BLAST analysis of the 853-bp fragment showed 99% identity with the sequences of various P. nicotianae isolates (e.g. Accession No. AF467087). Pathogenicity tests were performed by wound-inoculation with a cork borer of ten two-year-old potted blue Mediterranean fan palms. A mycelial plug of seven-day-old colonies grown on PDA was inserted into the basal stem and the hole was covered with the removed tissue and sealed with Parafilm^®. Control plants were inoculated with sterile agar plugs. All plants were incubated at 24 ± 1°C for 48 h with 100% relative humidity. After 30 days, all inoculated plants showed withering symptoms, whereas control plants remained healthy. P. nicotianae was re-isolated only from plants in which symptoms developed. Although P. nicotianae is one of the most frequent Phytophthora species in ornamental nurseries in Sicily (Pane et al., 2005), to our knowledge this is the first report of this Phytophthora species as a pathogen on C. humilis worldwide. It is conceivable that moistened soil due to excess of irrigation and heavy rainfall during hot season enhanced disease development in the nursery.