First Report of Petiole (Rachis) Blight of Washingtonia filifera Caused by Phoma glomerata in Greece
Authors: Ligoxigakis E. K. , Markakis E. A. , Papaioannou I. A. , Typas M. A. 
Published: June 2013

In July 2007, a severe petiole (rachis) blight disease was observed on several California fan palms (Washingtonia filifera) in the vicinity of Heraklion (Crete), Greece. Typical symptoms included discolored (brown to reddish-brown), reversed V-shaped lesions on the petiole bases of the oldest (lowest) leaves, and elongated yellow to dark-brown stripes along the petiole. The lesions progressively expanded and penetrated the petioles, resulting in gradual discoloration (from tan to brown-black) of the internal petiole tissues, including the vascular tissue. The bases of infected petioles occasionally became fragile and burst open, while the corresponding leaf blades were characterized initially by yellowing and one-sided or uneven wilt and, later, desiccation and death with the entire leaves curving downwards. The disease gradually moved upward to younger leaves, severely debilitating but rarely killing the infected trees. A filamentous fungus was consistently isolated onto potato dextrose agar (PDA) plates from sections of diseased petioles, forming dense, dark green colonies with abundant light to dark brown, subglobose pycnidia (diameter ranging between 36.4 to 177.4 μm, and averaging 99.4 μm, n = 50) on the agar surface or immersed in the medium. Chlamydospores and numerous dictyochlamydospores were also observed, with the latter being initially light to dark brown and later becoming black. The numerous conidia were hyaline, ovoid to ellipsoid, and single-celled. Their dimensions were 5.3 to 7.3 × 2.4 to 4.9 μm, averaging 6.5 × 3.2 μm (n = 100). The ITS1-5.8S-ITS2 region, together with parts of the flanking 18S and 28S rRNA genes (3), were amplified with PCR from total DNA extracted from two representative isolates, and sequenced (GenBank Accession Nos. KC802086 to KC802087). Using BLASTn, both sequences were 100% identical to Phoma glomerata ITS sequences (FJ427018, FJ427011, AF126816). Based on morphological and molecular analyses, the pathogen was identified as Phoma glomerata (Corda) Wollenw. & Hochapfel, also known as Peyronellaea glomerata (Corda) Goid. ex Togliani or Coniothyrium glomeratum Corda (1,2). To prove pathogenicity and fulfill Koch's postulates, petioles of the older leaves of eight W. filifera 2-year-old seedlings were wounded with a sterile scalpel (shallow cuts 0.5 to 1.0 cm wide, made parallel to the surface), inoculated with agar discs from a 2-week-old PDA culture of the fungus, and sealed with Parafilm. For controls, sterile PDA plugs were placed on the artificial wounds of five more seedlings. All plants were maintained in the greenhouse at 15 ± 5°C, with 90% humidity. Petiole blight and leaf necrosis symptoms—identical to those observed in the infected plants—were evident 5 weeks post-inoculation, and P. glomerata was consistently reisolated from all inoculated plants. No symptoms were observed on control plants. This is the first report of petiole blight of a palm species caused by P. glomerata in Greece. Due to the extensive use of palms as ornamentals in Greece, the occurrence of P. glomerata can potentially cause economic loss to the local ornamental industry. References: (1) M. M. Aveskamp et al. Stud. Mycol. 65:1, 2010. (2) R. M. Hosford, Jr. Phytopathology 65:1236, 1975. (3) M. P. Pantou et al. Mycol. Res. 109:889, 2005.