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First Report of Southern Blight Caused by Sclerotium rolfsii on Hemp (Cannabis sativa) in Sicily and Southern Italy

Hemp (Cannabis sativa L.), family Cannabaceae, is an annual herbaceous plant that is 1.5 to 4.0 m tall and native to the Caucasus Region, northern India, and Iran. It is cultivated in warm to temperate regions worldwide for its fiber, oil, and psychoactive substances. In Europe, commercial plantings have decreased from 52,872 ha in 1989 to 18,716 ha in 2005. Recently however, cultivation of hemp as a natural fiber species has been encouraged by European Union policy (2). During the summer of 2003, patches of dead plants were observed in test plots of 12 monoecious and dioecious hemp cultivars (Beniko, Epsylon 68, Felina 34, Ferimon, Fedora 17, Futura 75, Bialobrzeskie, Dioica 88, Fibranova, Tiborszallasi, Lovrin, and Carmagnola) in an experimental field near Catania (eastern Sicily) previously planted to artichoke (Cynara scolymus L.). Plots were irrigated with a drip irrigation system. Symptoms were first detected in July with day/night temperatures between 35 and 26°C. Infected plants showed a dark brown-to-tan discoloration of the stem near the soil line. As disease progressed, the rot extended down to the crown and taproot, foliage became yellow, and the entire plant eventually collapsed. An extensive white, cottony mycelium and numerous spherical tan-to-dark brown sclerotia (0.5 to 4.0 mm in diameter) developed externally on infected tissues and soil. As much as 60% of the plants were affected in a single plot. Monoecious cultivars that had been planted earlier escaped the disease. Isolations from diseased tissues were performed by plating symptomatic tissues previously disinfected for 1 min in 1% NaOCl and rinsed in sterile water on acidified potato dextrose agar (pH 4.5). Isolations consistently yielded a fungus whose characters corresponded to Sclerotium rolfsii Sacc. (teleomorph Athelia rolfsii (Curzi) Tu & Kimbrough). Pathogenicity of two isolates obtained from infected plants was confirmed by inoculating 120-day-old hemp plants grown in individual pots. Twenty plants for each of the above listed cultivars (10 plants for each isolate) were inoculated by applying toothpick tips (5 mm) colonized by S. rolfsii to the lower part of the stem. Ten noninoculated plants served as controls. Plants were kept in a greenhouse with temperatures between 26 and 32°C and 95% relative humidity. High soil moisture was maintained with frequent watering. All inoculated plants showed blight and basal stem rot after 2 weeks, irrespective of the cultivar. By the third week, plants began to dry up and mycelium and sclerotia developed on the crown. Noninoculated controls remained symptomless. S. rolfsii was reisolated from inoculated plants. Although S. rolfsii has been reported on hemp in India since the 1930s (3), to our knowledge, this is the first report of southern blight caused by this fungus on C. sativa in Sicily and southern Italy. Residues of artichoke, a very susceptible host of S. rolfsii (1), might have been the source of inoculum for this outbreak on hemp. Most likely, high summer temperatures and overirrigation exacerbated the disease severity.

References: (1) C. Cariddi and R. Lops. La Difesa delle Piante 19(1):27, 1996. (2) S. L. Cosentino et al. Agroindustria 2:137, 2003. (3) G. P. Hector. Ann. Rep. Dep. Agric. Bengal 35, 1931.