The genus Aeonium, family Crassulaceae, comprises approximately 35 species that are native to northern Africa and the Canary Islands. Tree aeonium (Aeonium arboreum (L.) Webb & Berthel.) is a bushy, perennial succulent with rosettes of tender, waxy leaves at the apex of few-branched or occasionally single, naked stems. Mature rosettes bear yellowish inflorescences. Aeoniums are cultivated as ornamentals in gardens and containers. During the summer of 2009, in a garden in eastern Sicily (southern Italy), 3-year-old potted plants of tree aeonium showed stunting, shrivelling, and chlorosis of leaves and drop of external leaves associated with root and basal stem rot. Drops of an amber exudate oozed from the basal stem. Tissues of the basal stem were soft, but no external necrosis was visible. A species of Phytophthora was consistently isolated from symptomatic roots and basal stem tissues on a medium selective for Oomycetes (2). Axenic cultures were obtained by single-hypha transfers. The pathogen was identified by morphological criteria as Phytophthora nicotianae B. de Haan; it formed stoloniferous colonies on potato dextrose agar and grew between 8 and 38°C, with the optimum at 30°C. On V8 juice agar it produced spherical, intercalary chlamydospores (mean diameter of 26 μm) and persistent, mono- and bipapillate, spherical to ovoid, ellipsoid, obpyriform sporangia that measured 29 to 56 × 22 to 45 μm with a mean length/breadth ratio of 1.3:1. All isolates were A2 mating type and formed spherical oogonia (mean diameter 28 ± 2 μm) with smooth walls and amphigynous antheridia in dual cultures with a reference isolate of the A1 mating type of P. nicotianae. BLAST analysis of the internal transcribed spacer (ITS) region of rDNA of a representative isolate from aeonium (IMI 398812, GenBank Accession No. HQ433333) amplified by PCR using the ITS6/ITS4 universal primers (1), revealed 99% similarity with the sequences of a reference isolate of P. nicotianae available in GenBank (Accession No. EU331089.1). Pathogenicity of isolate IMI 398812 was demonstrated by transplanting cuttings of A. arboreum into pots filled with a mixture of steam-sterilized sandy loam soil and inoculum (4% vol/vol) produced by growing the isolate for 20 days on wheat kernels. Ten plants were transplanted into 3-liter pots (two plants per pot) while 10 plants, transplanted into pots filled with a mixture of steam-sterilized soil and noninoculated kernels, were used as controls. Plants were kept in a greenhouse at 25 to 28°C and watered daily to field capacity. Thirty to forty days after the transplanting into infested soil, cuttings developed the same symptoms observed on plants with natural infections. Control plants remained symptomless. P. nicotianae was reisolated from symptomatic plants, thereby completing Koch's postulates. To our knowledge, this is the first report of P. nicotianae on an Aeonium species worldwide. The economic relevance of this disease is minor because aeoniums are not cultivated on a large scale. Moreover, the disease may be easily prevented by avoiding excess irrigation water since aeoniums need a well-drained soil or potting mix and do not tolerate soil waterlogging.
References: (1) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (2) H. Masago et al. Phytopathology 67:425, 1977.
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