During spring 2012, symptoms of an unusual leaf spot disease were observed in several commercial greenhouses near Salerno (southern Italy) on plants of Diplotaxis tenuifolia (cv Selvatica). The first symptoms on leaves of affected plants consisted of small (1 mm) black-brown spots of irregular shape, later coalescing into larger spots, 1 cm in diameter. Spots were surrounded by a yellow halo, and were mostly located on the foliar limb, rib, and petiole. Affected leaves were often distorted, appearing hook-like. The disease was severe under 75 to 90% RH, at air temperature of 20 to 26°C, and caused severe production losses on about 50 ha. Particularly, affected tissues rotted quickly after packaging and during transit and commercialization of processed rocket. Diseased tissue was excised, immersed in a solution containing 1% sodium hypochlorite for 60 s, rinsed in water, then placed on potato dextrose agar (PDA) medium, containing 25 mg/l of streptomycin sulphate. After 5 days, a fungus developed producing a whitish-orange mycelium when incubated under 12 h/day of fluorescent light at 22°C. The isolates obtained were purified on PDA. On this medium, they produced hyaline elliptical and ovoid conidia, sometimes one-septate, measuring 4.5 to 9.2 × 1.7 to 3.5 (average 6.8 × 2.6) μm. Conidia were born on phialides, measuring 6.8 to 20.2 × 1.3 to 3.1 (average 16.5 × 2.1) μm. Such characteristics are typical of Plectosphaerella sp. (2). The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS1/ITS4 (3) and sequenced. BLAST analysis of the 519-bp segment showed a 98% similarity with the sequence of Plectosphaerella cucumerina (GenBank Accession No. AB469880). The nucleotide sequence has been assigned the GenBank Accession JX185769. To confirm pathogenicity, tests were conducted on 45-day-old D. tenuifolia plants. Plants (21/treatment), grown in 15 liter pots (7 plants/pot) were inoculated by spraying a 1 × 106 CFU/ml conidial suspension of one isolate of P. cucumerina, prepared from 10-day-old cultures, grown on PDA. Inoculated plants were maintained in a growth chamber at 23 ± 1°C, at 90% RH for 4 days. Non-inoculated plants served as control. Inoculated plants showed the typical first leaf spots 6 days after the artificial inoculation. Four days later, spots enlarged and leaves became distorted, showing chlorosis. Non-inoculated plants remained healthy. P. cucumerina was reisolated from inoculated plants. The pathogenicity test was conducted twice with identical results. This is, to our knowledge, the first report of P. cucumerina on D. tenuifolia in Italy as well as worldwide. P. cucumerina has been described as associated with root and collar rots of other horticultural crops in southern Italy (1). Due to the importance of the crop in Italy, this disease can cause serious economic losses.
References: (1) A. Carlucci et al., Persoonia, 28:34, 2012. (2) M. E. Palm et al. Mycologia, 87:397, 1995. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.
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