Blight Caused by Sclerotium rolfsii on Potted Ornamental Citrus in Sicily.

Abstract

Approximately 140,000 container-grown ornamental citrus plants are produced each year in the province of Catania (eastern Sicily). In the spring of 2006, a severe blight was observed in a commercial nursery in Catania on 2-month-old rooted cuttings of lemon (Citrus limon (L.) Burm.) and calamondin (× Citrofortunella mitis (Blanco) J. W. Ingram & H. E. Moore). Approximately 80% of the nursery stock of 2,000 cuttings was affected. Cuttings were grown in 7.5-cm2 pots made with compressed peat and wood pulp at 28 to 30°C with 95 to 100% relative humidity on benches in a greenhouse, The pot mix was composed of peat, perlite, and soil (2:1:2). Cuttings showed a dark brown necrotic lesion at the base of the stem that extended upward, resulting in chlorosis and wilting of the leaves. An invasive, white, cottony mycelium with a fan-like pattern and numerous, small, brown spherical sclerotia (0.5 to 4.0 mm in diameter) developed on infected tissues, in the potting mix as well as on the pot wall. Herbaceous cuttings collapsed within 2 weeks while woody cuttings gradually died. Symptomatic basal stem sections were disinfected for 1 min in 1% NaOCl, rinsed in sterile water, and plated on acidified (pH 4.5) potato dextrose agar (PDA). Isolations consistently yielded a fungus whose morphological characters corresponded to Sclerotium rolfsii Sacc. On PDA, it produced a septate mycelium with clamp connections and numerous olive brown-to-clove brown sclerotia (1 to 3 mm in diameter). Pathogenicity of two S. rolfsii isolates (IMI 396204 and IMI 396205) from citrus was confirmed on 3-month-old lemon cuttings grown in 10-cm-diameter plastic pots filled with a sterilized mix of peat moss and vermiculite (3:1) (10 cuttings for each isolate). Each pot was inoculated with 15 sclerotia harvested from 6-week-old cultures on PDA and placed on the soil surface around the base of the cutting. Ten noninoculated cuttings served as the control. Cuttings were kept in a growth chamber at 28°C and relative humidity at >95%. All inoculated cuttings showed wilting, blight, and stem rot within 3 weeks after inoculation. White mycelium and sclerotia were produced on the stem base and soil surface. Noninoculated controls remained symptomless. S. rolfsii was reisolated from infected cuttings. The pathogenicity test was repeated once with calamondin cuttings and the results were similar. Blight caused by S. rolfsii is widespread in nurseries of ornamentals in Italy (1). However, to our knowledge, this is the first report of this disease on potted ornamental citrus. Probably high temperature and moisture during rooting were conducive to the disease.