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First Report of Leaf Spot, Blight, and Stem Lesions Caused by Cylindrocladium pauciramosum on Callistemon in Spain

Callistemons (Callistemon citrinus (Curtis) Skeels), evergreen plants of the family Myrtaceae, are commonly grown in Mediterranean gardens for their red bottlebrush-like flowers. During November of 2006, 1-year-old potted plants of callistemon showed leaf spots and blight in commercial nurseries in Valencia, Spain. Symptoms consisted initially of minute brown spots on the leaves, developing into black-gray blotches that finally coalesced. Diseased plants also showed stem lesions and blight of young shoots. Approximately 30% of the plants were affected. A Cylindrocladium sp. was isolated consistently from the infected tissues. Six single conidial isolates were grown on carnation leaf agar (CLA) under near-UV light at 25°C for 7 days (1). The macroconidiophores comprised of a stipe, a sterile elongation, and a penicillated arrangement of primary, secondary, and tertiary branches. The stipes were septate, 110 to 175 (138) μm long, with a terminal obpyriform vesicle measuring 3.75 to 7.5 (5.8) μm wide. Phialides (12.5 × 3.6 μm) were hyaline, doliiform to reniform, with conidia 40 to 55 × 3.7 to 5 μm, cylindrical with rounded ends, aseptate or one septate. Chlamydospores were brown and formed microsclerotia. These features conformed to the description of Cylindrocladium pauciramosum (3). Further confirmation was obtained by sequence analysis. The 5′ end of the β-tubulin gene was amplified using primers T1 and βt2b (2). Comparison with other sequences in GenBank revealed that the isolates described here were identical with C. pauciramosum (Accession No. AY880064) isolated from Ceanothus in the UK. To confirm pathogenicity, 1-year-old plants of callistemon were inoculated with two isolates by spraying with a spore suspension of the fungus (1 × 10⁵ conidia per ml) obtained from 14-day-old single spore colonies on CLA. Control plants were treated with sterile distilled water. After inoculation, all plants were maintained in plastic bags and kept at 22 ± 2°C. Four days after inoculation, the plants developed symptoms similar to those observed in natural infections, and C. pauciramosum was reisolated, successfully completing Koch's postulates. No symptoms were observed on the control plants. C. pauciramosum has been recorded on several hosts, including Callistemon citrinus, in Italy (4). To our knowledge, this is the first report of C. pauciramosum on callistemon in Spain.

References: (1) P. W. Crous and M. J. Wingfield. Mycotaxon 51:341, 1994. (2) B. Henricot and A. Culham. Mycologia 94:980, 2002. (3) C. L. Schoch et al. Mycologia 91:286, 1999. (4) C. L. Schoch et al. Plant Dis. 85:941, 2001.