First Report of Sirosporium celtidis Causing a Foliar Disease of European Hackberry in Spain.

Abstract

Hackberry (Celtis australis L.) is widely used for reforestation and as shade tree in parks and roadside plantings in southern Europe (4). In autumn 2011, a foliar disease was observed affecting several trees planted in a garden area located in Alzira (Valencia province, eastern Spain). Symptoms appeared on lower leaf surfaces as reddish to dark brown velvety irregular spots, later becoming grayish brown on the upper surface. Most of the infected trees were prematurely defoliated. Spots on lower leaf surfaces were covered by mycelium, conidiophores, and conidia. Fungal isolates were recovered directly from the structures present on the lesions and by surface-disinfecting small fragments of symptomatic leaf tissue in 0.5% NaOCl, double-rinsing the sections in sterile water, and plating the sections onto potato dextrose agar (PDA) amended with 0.5 g of streptomycin sulfate per liter. Single conidium cultures made onto PDA were maintained for 2 months at 25°C in darkness for morphological examination. Conidia were thick walled, dark reddish brown, often markedly curved or coiled, cylindrical to obclavate, smooth, wrinkled, or verrucose, typically multicellular, 2 to 40 transversely septate and occasionally with 1 to 3 longitudinal or oblique septa that were often constricted, 20 to 96 (44.9) × 6 to 9 (7.1) μm, with an inconspicuous scar at the base. Morphological characters corresponded to the description of Sirosporium celtidis (Biv. ex Spreng) M. B. Ellis published in 1963 (3). The internal transcribed spacer (ITS) region of the rDNA was amplified with the primers ITS1 and ITS4 from DNA extracted from the isolate AL1, and sequenced (GenBank Accession No. JX397963). The sequence was identical to that obtained from an isolate of S. celtidis from the Centraalbureau voor Schimmelcultures, Utrecht, The Netherlands (CBS 289.50). Pathogenicity tests were conducted on five 2-year-old hackberry trees by spraying onto the upper and lower leaf surfaces a conidial suspension of S. celtidis (approximately 50 ml/plant, 106 conidia/ml of water). Five control plants were sprayed with sterile water. Plants were covered with clear plastic bags and incubated in a growth chamber for 72 h at 25°C with a 12-h photoperiod. First leaf spots were visible on inoculated plants after 7 days, but symptoms were not observed on control plants. The fungus was reisolated from leaf lesions on inoculated plants, confirming Koch's postulates. S. celtidis was first described in Sicily in 1815 (3) and has been recorded on various hackberry species in Mediterranean countries and the USA (1,2). To our knowledge, this is the first report of the disease in Spain. The economic and ecological significance of the pathogen in natural ecosystems in Spain remains to be determined but it could certainly become a serious problem for nurseries and urban plantings.