First Report of Leaf Spot Caused by Corynespora cassiicola on Basil (Ocimum basilicum) in Italy

Abstract

Sweet basil (Ocimum basilicum) is an economically important herb in several Mediterranean countries. Approximately 80 ha are grown annually in Italy for fresh consumption and processing. In 2006, a damaging foliar disease of sweet basil cv. Genovese gigante was observed in several greenhouses located in the Liguria Region of northern Italy. A disease incidence of more than 50% was observed. Leaves of infected plants initially showed dark brown-to-black, 1 to 3 × 1.5 to 5.3 mm in diameter, circular spots surrounded by a chlorotic halo. Within 2 to 3 days, the spots coalesced, leading to extensive leaf necrosis with occasional shot holes. Stem lesions were brown, elongated, and irregularly dispersed. Although the distribution of the disease generally became uniform, it usually appeared originally in a patchy pattern at the central areas of the greenhouses, where temperatures and relative humidity were highest. Basal leaves were most severely affected by the disease when air circulation was apparently poor. Microscopic observations revealed light brown conidiophores, ending in sterigmata with conidia borne singly or in chains. Conidia were 31.9 to 212.2 μm (average 68.1 μm) long and 4.3 to 11.6 μm (average 7.8 μm) wide, with longitudinal cross walls. Such morphology is typical of Corynespora cassiicola (3). The fungus was consistently isolated from symptomatic leaves onto potato dextrose agar. The ITS region (internal transcribed spacer) of rDNA was amplified using primers ITS4/ITS6 and sequenced. BLASTn analysis (1) of the 215 bp obtained showed an E-value of 0.0 with C. cassiicola and the nucleotide sequence has been assigned GenBank Accession No. EF 545008. Pathogenicity tests were performed by spraying leaves of 10 healthy 30-day-old potted O. basilicum plants cv. Genovese gigante with a 105 conidia per ml aqueous suspension. Plants in 10 pots sprayed with water only served as controls. Plants were covered with plastic bags for 24 h after inoculation and maintained at 20 to 25°C. The first foliar lesions developed on leaves 3 to 4 days after inoculation, whereas control plants remained healthy. C. cassiicola was consistently reisolated from these lesions. The pathogenicity test was completed twice. To our knowledge, this is the first report of C. cassiicola on O. basilicum in Italy. Other previous records of this disease were from India (2) and Brunei (4). References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) L. R. Devi et al. Indian Phytopathol. 32:150, 1979. (3) M. B. Ellis. CMI Mycol. Pap. No. 65, 1957. (4) W. T. H. Peregrine and K. B. Ahmad. Phytopathol. Pap. 27:1, 1982.