Turnip (Brassica rapa subsp. rapa L.) is an annual vegetable crop cultivated for consumption of its succulent root. In July 2011, symptoms consisting of leaf spots 1 to 8 mm in diameter with a dark brown color surrounded by a yellow halo and black sunken lesions in the swollen storage root were observed in production areas in Alicante Province in east-central Spain. Disease incidence was approximately 20% in fields of about 3 ha where infection was highest. Symptomatic leaves and roots collected from turnip cv. Virtudes-Martillo in three affected fields were surface disinfected with 0.5% NaOCl for 2 min, and small fragments from necrotic lesions were plated on potato dextrose agar (PDA) amended with 0.5 g streptomycin sulfate per liter. Alternaria colonies were consistently isolated from affected leaves and roots after 7 days of incubation at 24°C, and were transferred to V-8 with autoclaved turnip cv. Virtudes-Martillo leaves. Two isolates from leaves and two isolates from roots were included in the study. Plates were incubated for 15 days at 24°C with an 8-h fluorescent light period and a 12-h dark period for morphological examination. Conidia produced in culture were mostly solitary or in short chains of 2 to 3 spores, beakless, ovoid to ellipsoid, and light brown. Conidia were 32 to 78 × 13 to 24 μm, with 3 to 7 transverse septa and 1 to 2 longisepta. Aggregated hyphal chains of dark, thick-walled ornamented cells distinctive of Alternaria japonica Yoshii (3) were observed. The 5.8S, ITS2, and 28S ribosomal RNA (rRNA) regions were amplified using the primers ITS3 and ITS4 (4) and sequenced from DNA extracted from the isolate designated as IVIA-A070, obtained from turnip leaves cv. Virtudes-Martillo in Alicante Province (GenBank Accession No. JX983044). The sequence had 100% identity (total score 302, 73% coverage) with that of A. japonica strain ATCC 13618 (2) (AY376639). Pathogenicity tests were performed twice on two 3-month-old plants of turnip cv. Virtudes-Martillo and cv. Blanco-Globo, and cabbage (B. oleracea var. capitata L.) cv. Brunswick. Plants were inoculated by spraying a conidial suspension of the isolate IVIA-A070 (10 ml/plant, 104 conidia/ml water) using manual pressure sprayer. Two plants of each host sprayed with sterile distilled water were used as controls in each experiment. Plants were covered with black plastic bags and incubated in a growth chamber for 48 h at 25°C. Leaf spots similar to those observed in affected plants in the field were visible on all turnip and cabbage plants 4 days after inoculation with the fungus. No symptoms were observed on control plants. Fungal colonies morphologically identified as A. japonica were reisolated from leaf lesions on inoculated turnip and cabbage plants, but not from asymptomatic leaves of control plants. Based on these results, the disease was identified as black spot of turnip caused by A. japonica. In Spain, black spot of brassicas was previously associated only with A. brassicae (Berkeley) Saccardo and A. brassicicola (Schw.) Wiltshire (1).
References: (1) P. Melgarejo et al. Patógenos de Plantas Descritos en España. MARM-SEF, Madrid, 2010. (2) B. M. Pryor and R. L. Gilbertson. Mycol. Res. 104:1312, 2000. (3) E. G. Simmons. Alternaria: An Identification Manual. CBS Fungal Biodiversity Centre, Utrecht, 2007. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.