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First Report of Crown Rot Caused by Cylindrocladium pauciramosum on Scarlet Honey Myrtle in Italy

Scarlet honey myrtle (Melaleuca fulgens R. Br.), native to Australia, is an evergreen colorful shrub (Myrtaceae) and grown in Italy as an ornamental plant. During November 2008, a widespread disease was noticed on ~90% of 3,000 6-month-old M. fulgens cv. Red potted plants. Plants were obtained from cuttings and produced by a commercial nursery in Catania Province. Symptomatic plants showed a crown rot and longitudinal sections of tissues revealed a brown discoloration of the basal stem. As a consequence, leaves gradually became necrotic and abscised, followed by death of the entire plant. Root rots and leaf spots were not observed. M. gibbosa, M. ericifolia, M. thymifolia, and M. elliptica, cultivated in the same nursery, did not show disease symptoms. A Cylindrocladium sp. was consistently isolated from the crown and basal stem of symptomatic plants on potato dextrose agar (1). Ten Cylindrocladium isolates obtained from infected basal stems and crowns were selected and cultured for 8 days at 25°C on carnation leaf agar (CLA). Macroconidiophores consisted of a stipe, a penicillate arrangement, and a stipe extension terminating in an obpyriform to ellipsoidal vesicle (6 to 10 μm in diameter). Cylindrical conidia were rounded at both ends, straight, 1-septate, and 42 to 60 × 4 to 5 μm. All single-conidial isolates were mated with opposite tester strains of C. pauciramosum on CLA and produced fertile perithecia (3). Perithecia were solitary or in groups, orange to red-brown, subglobose to ovoid, and 270 to 400 μm high × 180 to 290 μm in diameter. Further confirmation of species was obtained by amplification and sequencing of the intergenic spacer (IGS) region of rDNA with the M13 forward (-20) and M13 reverse primers. On the basis of the complete IGS sequence, two primer sets (218F/218R and 106F/106R) were designed and successfully used in a nested-PCR protocol for the detection of C. pauciramosum from tissues of infected plants (2). On the basis of morphological characters, mating type, and molecular data, the isolates were identified as C. pauciramosum C.L. Schoch & Crous. One representative isolate (DISTEF-MFR2; CBS 124657) was deposited at Centraalbureau voor Schimmelcultures open fungi collection (Fungal Biodiversity Centre, Utrecht, the Netherlands). Pathogenicity tests were performed by adding sterile water to CLA cultures of C. pauciramosum from a single-conidial isolate and incorporating the resulting spore suspension (10⁵ conidia per ml) on the soil surface of 20 3-month-old M. fulgens cv. Red potted plants. The same number of plants served as uninoculated controls. Following inoculation, plants were well irrigated and maintained in a growth chamber at 25 ± 1°C and 90 to 95% relative humidity. All inoculated plants developed crown rot symptoms identical to those observed in the nursery 2 months after inoculation. Control plants remained symptomless. C. pauciramosum was always reisolated from the infected plants and identified as previously described. C. pauciramosum was previously detected in Italy as being responsible for a leaf spot on M. hypericifolia (3). To our knowledge, this is the first record in the world of crown rot of scarlet honey myrtle caused by C. pauciramosum.

References: (1) P. W. Crous. Taxonomy and Pathology of Cylindrocladium (Calonectria) and Allied Genera. The American Phytopathological Society, St. Paul MN, 2002. (2) F. Nigro et al. J. Plant Pathol. 88:S22, 2006. (3) G. Polizzi and P. W. Crous. Eur. J. Plant Pathol. 105:407, 1999.