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First Report of Southern Blight on Silverbush (Convolvulus cneorum) Caused by Sclerotium rolfsii in Italy

The genus Convolvulus has more than 200 species that are encountered in temperate to tropical climates all around the world. Convolvulus cneorum L., also known as silverbush, is a perennial shrub native to southern Europe (Sicily and Croatia) with dense, silver foliage and masses of large, circular, white flowers. During July of 2009, a widespread blight was observed on approximately 10% of 12,000 4-month-old potted silverbush plants. The plants were obtained from cuttings and produced by a commercial nursery in eastern Sicily, Italy. Symptomatic plants initially had sunken, tan lesions at the ground level that developed into typical southern blight. Circular and crescent-shaped patches were observed on the masses of weeds on the surface of the containers where silverbush were grown. At the soil line, white mycelia and small (1 to 2 mm in diameter), brown, spherical sclerotia with internally differentiated rind, cortex, and medulla characteristic of Sclerotium rolfsii Sacc. were observed. Crown and stem lesions were surface disinfested (1% NaOCl) for 1 min, rinsed in sterile water, and placed on potato dextrose agar. Isolation consistently yielded colonies of Sclerotium rolfsii (teleomorph Athelia rolfsii (Curzi) Tu & Kimbrough) with typical sclerotia produced within 6 to 7 days (2). Pathogenicity tests were performed on 20 plants by placing 10 sclerotia obtained from 10-days-old cultures in the soil below the crown portion on each of 5-month-old healthy cuttings of silverbush. The same number of plants served as noninoculated controls. All plants were maintained in a growth chamber at 25 ± 1°C and enclosed for 7 days in polyethylene bags. Plants were then moved to a greenhouse where temperatures ranged from 24 to 28°C. Symptoms of southern blight developed after 7 to 20 days on all inoculated plants. Control plants remained symptomless. S. rolfsii was reisolated from symptomatic plants. S. rolfsii was reported for the first time in Sicily in 2004 in an ornamental nursery (1). To our knowledge, this is the first report in the world of S. rolfsii on silverbush and it is the first outbreak of southern blight on Convolvulus species. The high susceptibility of silverbush to the pathogen could be a limiting factor for the cultivation in nursery of this indigenous wildflower plant.

References: (1) G. Polizzi et al. Plant Dis. 88:310, 2004. (2) Z. K. Punja and A. Damiani. Mycologia 88:694, 1996.