First report of a leaf spot of sweet basil (Ocimum basilicum) caused by Alternaria alternata in Italy.
During the summer-autumn of 2010, extensive necroses were observed on the leaves of sweet basil (O. basilicum) plants grown in soilless systems and in the soil in Piedmont (northern Italy). The disease affected 10% of 60-day-old plants in soilless systems and 40% of 5-month-old plants grown in the soil. Brown-black lesions often surrounded by a yellow halo developed from the margins and tips of the upper side of older leaves, leading to the progressive defoliation of plants, rarely followed by plant death. A fungus, consistently isolated on PDA from symptomatic leaves, formed conidia singly or in short chains (2-8 elements), dark brown, with 3-7 transverse and 0-4 longitudinal septa, 23.7-73.4 × 8.8-15.1 µm in size, and with a conical or cylindrical beak 3.5-19.4 µm long. The pathogen was identified as an Alternaria sp. based on morphology. The ITS region of rDNA was amplified using ITS1/ITS4 primers and sequenced. BLAST analysis of a 523-bp segment confirmed that the sequence corresponded to A. alternata (GenBank accession no. HQ540552). This is thought to be the first report of A. alternata in O. basilicum from Italy and Europe.