First Report of Collar and Root Rot Caused by Phytophthora nicotianae on Oriental Paperbush (Edgeworthia papyrifera) in Italy

Abstract

Edgeworthia papyrifera, Oriental paperbush, is a deciduous flowering shrub becoming increasingly popular because of its clove-like perfumed flowers appearing in late winter-early spring. During August of 2009 in a commercial nursery close to Maggiore Lake (Verbano-Cusio-Ossola Province) in northwest Italy, 2-year-old plants of E. papyrifera showed extensive chlorosis and root rot. Twigs wilted and died, dropping leaves in some cases. Most frequently, wilted leaves persisted on stems. At the soil level, dark brown-to-black water-soaked lesions formed and coalesced, girdling the stem. All of the crown and root system was affected. Infected plants died within 14 days of the appearance of symptoms. Disease was widespread and severe, affecting 90 of the 100 plants present. After disinfestation for 1 min in a solution containing 1% NaOCl, rotting root and collar pieces of E. papyrifera consistently produced a Phytophthora-like organism when plated on a medium selective for oomycetes (3). The pathogen was identified morphologically as Phytophthora nicotianae (= P. parasitica) (2). On V8 agar, coenocytic hyphae, 4 to 8 μm in diameter, formed fluffy, aerial colonies and spherical, intercalary chlamydospores, 21.0 to 36.5 (average 26.7) μm in diameter. Colonies grew well at 35°C and stopped growing at 40°C. Sporangia were produced by growing a pure hyphal-tip culture in a diluted, sterilized soil-extract. Sporangia were borne singly, laterally attached to the sporangiophore, were noncaducous, spherical to ovoid, papillate, and measured 28.6 to 55.2 × 22.4 to 45.1 (average 42.4 × 34.6) μm, length/breadth ratio (1.1:1)-1.2:1-(1.3:1). Papillae measured 3.1 to 7.6 (average 4.6) μm. The internal transcribed spacer (ITS) region of rDNA of a single isolate was amplified with primers ITS4/ITS6 and sequenced. BLAST analysis (1) of the 839-bp segment showed 99% homology with the sequence of P. nicotianae (No. AJ854296). The sequence has been assigned the GenBank No. GU353341. Pathogenicity of isolates Edg.1 and Edg.2 obtained, respectively, from the root and collar of an infected plant was confirmed by inoculating 1-year-old plants of E. papyrifera. Both strains were grown for 15 days on a mixture of 70:30 wheat/hemp kernels, and 4 g/liter of the inoculum was mixed into a substrate containing sphagnum peat moss/pumice/pine bark/clay (50:20:20:10 vol/vol). One plant per 3-liter pot was transplanted into the substrate and constituted the experimental unit. Five plants were used for each test strain and noninoculated control treatment; the trial was repeated once. All plants were kept in a greenhouse at 25 to 28°C. Plants inoculated with Edg.1 and Edg.2 developed chlorosis and root rot 18 and 14 days after the inoculation, respectively, and wilt rapidly followed. Control plants remained symptomless. P. nicotianae was consistently reisolated from inoculated plants. To our knowledge, this is the first report of P. nicotianae on E. papyrifera in Italy as well as worldwide. The current economic importance of the disease is minor due to the limited number of farms that grow this crop in Italy, although spread could increase as the popularity of plantings expand.