Stevia rebaudiana (sweetleaf) is a perennial shrub belonging to the Asteraceae family and is widely grown for its sweet leaves. With its extracts having as much as 300 times the sweetness of sugar, this species is used in many countries for the production of sugar substitutes. However, in Italy, as well as in other countries, this species cannot be grown for the use of its leaf extracts. This plant is grown in a few nurseries in the Albenga Region (northern Italy) as potted plants. In February of 2008, 3-month-old plants grown in plastic pots (14-cm diameter) under glasshouse on heated benches started showing symptoms of a previously unknown blight. The temperature in the glasshouse ranged between 16 and 20°C and plants were watered by sprinkle irrigation. Leaves, starting from the basal ones, showed small, brown spots that spread across the entire leaf surface. Subsequently, the crown and stem were infected, and the pathogen developed abundant, soft, gray mycelium on leaves and stems and in the middle of the heads of S. rebaudiana. Flowers were not present when the symptoms appeared. Severely infected leaves dried out and became necrotic. The disease was observed in one nursery in which 5% of the plants were affected. The margins of the lesions were excised from leaves, immersed in a solution containing 1% sodium hypochlorite, and then cultured on potato dextrose agar (PDA) medium. A fungus produced abundant mycelium when incubated under constant fluorescent light at 22 ± 1°C after 10 days. The conidia were smooth, hyaline, ovoid, measuring 15.5 to 8.3 × 11.1 to 7.3 (average 11.6 × 8.6) μm, and were similar to those described for Botrytis cinerea. Conidiophores were slender and branched with enlarged apical cells bearing conidia on short sterigmata. The identity of the fungus was also confirmed by the production of numerous, small, black sclerotia on PDA plates incubated for 20 days at 8 ± 1°C. Sclerotia were dark and irregular with a diameter ranging from 1 to 2 mm. These morphological characters identified the fungus as B. cinerea (2). The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS4/ITS6 and sequenced. BLAST analysis (1) of the 780-bp segment showed a 100% homology with the sequence of Botryotinia fuckeliana (perfect stage of B. cinerea). The nucleotide sequence has been assigned GenBank Accession No. FJ486270. Pathogenicity tests were performed by spraying leaves of six healthy 6-month-old potted S. rebaudiana plants with a 105 conidia/ml suspension. Six plants sprayed with water only served as controls. Plants were covered with plastic bags for 3 days after inoculation to maintain high relative humidity and were placed in a growth chamber at 20 ± 1°C. The first foliar lesions developed on leaves 4 days after inoculation, whereas control plants remained healthy. B. cinerea was consistently reisolated from these lesions. The pathogenicity test was completed twice. To our knowledge, this is the first report of the presence of B. cinerea on S. rebaudiana in Italy. The disease has been reported in Ukraine (3) and more recently in Japan (4). The economic importance of this disease is at the moment limited.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) H. L. Barnett and B. B. Hunter. Illustrated Genera of Imperfect Fungi. Burgess Publishing Company, Minneapolis, MN, 1972. (3) J. Takeuch and H. Horie. Annu. Rep. Kanto-Tosan Plant Prot. Soc. 53:87, 2006. (4) V. F. Zubenko et al. Zash. Rast. 18, 1991.
