First Report of Leaf Spot and Shoot Blight Caused by Cylindrocladium scoparium on Mallee Honeymyrtle in Italy

In December of 2008, a widespread disease was observed on several blocks of approximately 15,000 plants (6-month to 2-year-old) of mallee honeymyrtle (Melaleuca acuminata F. Muell.). The plants were grown in two nurseries in eastern Sicily where high diffusion of diseases caused by Cylindrocladium pauciramosum and C. scoparium was previously detected. The plants exhibited leaf spots, defoliation, and apical blight of shoots. Crown rot and root rot were not present. Leaf spots were detected on all plants, whereas shoot blight was observed on approximately 3% of the plants. A Cylindrocladium sp. was consistently isolated from the diseased portions of plants onto potato dextrose agar. To determine the species, 20 single-conidia isolates of the fungus obtained from symptomatic tissues from different blocks and nurseries were cultured on carnation leaf agar (CLA) for 7 days at 25°C under 12-h light/dark conditions. Mycelia and spores growing on the carnation leaves were examined with a light microscope and the isolates were identified as C scoparium Morgan (teleomorph Calonectria morganii Crous, Alfenas & M.J. Wingf.) on the basis of their pyriform to broadly ellipsoidal terminal vesicles, conidiophore branching pattern, and conidia (1). In addition, the ability of the isolates to mate with South African and Italian opposite tester strains of C. scoparium (2,4) confirmed the identification of all the isolates. Koch's postulates were fulfilled by inoculating 30 6-month-old plants of mallee honeymyrtle with a spore suspension (105 conidia per ml) of one isolate of the pathogen (DISTEF-MA1) obtained from 14-day-old single-spore colonies grown on CLA at 24°C under fluorescent cool white lights on a 12-h light/dark regimen. Following inoculation, all plants were maintained in plastic bags in a growth chamber at 25 ± 1°C and 90 to 95% relative humidity. The same number of mallee honeymyrtle plants was used as uninoculated controls. Leaf spots, defoliation, and apical shoot blight identical to those observed in the nurseries appeared within 4 to 25 days. No symptoms were detected on the control plants. C. scoparium was reisolated from the artificially infected tissues and was identified as previously described. The isolate used in the pathogenicity proof was deposited at the Fungal Biodiversity Centre, Centraalbureau voor Schimmelcultures (Accession No. CBS 124658). In Italy, C. scoparium was detected for the first time on Pistacia lentiscus in 2005 (3). Another report confirmed the spread of the pathogen in Sicilian ornamental nurseries (4). To our knowledge, this is the first record in the world of C. scoparium causing disease on mallee honeymyrtle. The data demonstrate the high susceptibility of this species to the C. scoparium leaf spot and twig blight especially when environmental conditions (heavy rains and mild temperatures) are conductive to the infections. References: (1) P. W. Crous. Taxonomy and Pathology of Cylindrocladium (Calonectria) and Allied Genera. The American Phytopathological Society, St. Paul MN, 2002. (2) P. W. Crous and M. J. Wingfield. Mycotaxon 51:341, 1994. (3) G. Polizzi et al. Plant Dis. 90:1110, 2006. (4) G. Polizzi et al. Plant Dis. 91:769, 2007.