First Report of Diplodia scrobiculata Causing Canker and Branch Dieback on Strawberry Tree (Arbutus unedo) in Italy.

Strawberry tree, family Ericaceae, is an evergreen shrub or small tree that grows in the Mediterranean Region. In spring 2009, a survey was conducted to study the fungi associated with canker and branch dieback of strawberry tree in a natural stand located on Caprera Island (41°12'N, 9°27'E), Italy. Fungal isolates obtained from live twigs and branches showing sunken necrotic bark lesions were identified as Diplodia scrobiculata J. de Wet, Slippers & M.J. Wingf. on the basis of morphological features (1). On potato dextrose agar (PDA) at 25°C, D. scrobiculata isolates developed white colonies with appressed mycelium that became dark gray after 4 to 6 days and formed pycnidia after 2 weeks on sterile Pinus radiata needles placed on the PDA surface. The conidia were dark brown with zero to three septa, clavate with truncate base, and measured 31.1 to 41.9 (36.8) × 11.3 to 16.3 (12.6) μm, with a length/width ratio of 2.9 ± 0.1 (mean ± standard error) (n = 50). Identity was confirmed by analysis of the internal transcribed spacer region (ITS1-5.8S-ITS2) of rDNA. BLAST searches in GenBank showed 100% similarity with reference sequences of D. scrobiculata (GenBank Nos. AY160200, EU220438, EU220444, and EU392283). The representative sequence of one D. scrobiculata strain (BL5), stored in the culture collection of the Department of Plant Protection at the University of Sassari, was deposited in GenBank (Accession No. GU722102). Pathogenicity of strain BL5 was tested by stem inoculation on eight 2-year-old strawberry tree seedlings maintained in a greenhouse at 18 to 26°C. A mycelial plug (3 to 4 mm) taken from the margin of an actively growing colony on PDA was put in a shallow wound (~3 mm) made by a scalpel on the basal part of the stem of each seedling. Four weeks after inoculation, the seedlings displayed dark brown-to-black discoloration, measuring 1.7 ± 0.6 cm (mean ± standard error) of the bark and wood tissues of the stems. The pathogen was successfully reisolated from symptomatic stem tissues, thus fulfilling Koch's postulates. Three control seedlings inoculated with sterile PDA plugs remained asymptomatic. These results demonstrate the active role played by D. scrobiculata in the aetiology of the canker and branch dieback observed on strawberry tree. D. scrobiculata is generally recognized as a weak pathogen of gymnosperms worldwide (2), however, it has recently been reported on olive in Italy (3). To our knowledge, this is the first report of D. scrobiculata on strawberry tree. Currently, further investigations are in progress to determine the possible role of biotic and abiotic factors in the development of this disease. References: (1) J. De Wet et al. Mycol. Res. 107:557, 2003. (2) J. De Wet et al. Mol. Phylogenet. Evol. 46:116, 2008. (3) C. Lazzizera et al. Fungal Divers. 31:63, 2008.