Leaf Spot of Arugula, Caused by Alternaria japonica, in California
Authors: Tidwell T. E. , Blomquist C. L. , Rooney-Latham S. , Scheck H. J. 
Published: August 2014

Arugula (Eruca vesicaria subsp. sativa (Mill.) Thell. is a Cruciferous plant used for culinary purposes. From 2012 to 2013, a foliar disease seriously impacted the growth and quality of about 0.1 ha of hydroponically grown arugula at a Santa Barbara County nursery. Samples of affected arugula seedlings exhibited adaxial and abaxial symptoms of mottling with circular to oval, water soaked, dark green leaf spots, each 1 to 3 mm in diameter, and some of which coalesced. Conidia of an Alternaria sp. were observed on the foliage. Symptomatic leaf pieces were disinfested with 0.6% NaOCl, blotted dry, and plated on acidified potato dextrose agar (APDA). Cultures were incubated under near-UV lights for 24 h/day. Olivaceous-grey colonies of the same Alternaria species observed on the leaves grew after 7 days. After 21 days on carrot-piece agar (3), the fungus produced beakless conidia with longitudinal and constricted transverse septa that measured 30.0 to 69.0 × 12.5 to 20.0 μm and were borne singly or in short chains of 2 to 3 conidia. In addition, knots of dark, thick-walled micro-chlamydospores were produced by the hyphae. The fungus was identified morphologically as Alternaria japonica Yoshii (2), and the species confirmed by sequence analysis. A portion of the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) was amplified using ITS1 and ITS4 primers (4). The sequence (GenBank Accession No. KJ126846) was 100% identical to the ITS rDNA sequence of an isolate of A. japonica (KC584201) using a BLASTn query. A. japonica was also detected in seeds of the lot used to grow the affected arugula crop. Pathogenicity of a single isolate was tested by inoculating four 37-day-old plants each of arugula, cabbage (Brassica oleracea L. var. capitata), and broccoli (B. oleracea L. var. botrytis L.). Inoculum was obtained from 11-day-old cultures of the isolate grown at 24°C on half-strength APDA. Half of a 2.5 cm diameter agar plug containing hyphae and conidia was ground in 2 ml of sterilized water, and the volume of water increased to 45 ml. Leaves of four plants/host species were sprayed with 3.5 to 4.0 ml of inoculum. The inoculated plants and four control plants of each species treated similarly with sterilized water were immediately incubated in a dark dew chamber at 23°C. After 72 h in the dew chamber, inoculated plants of all three hosts produced similar symptoms of wilting, water soaking, and dark green leaf spotting as the original symptomatic field plants. Conidia formed in the leaf spots on both sides of inoculated leaves. A. japonica was re-isolated from all of the inoculated plants but from none of the symptomless control plants using the method previously described. Pathogenicity tests were repeated, with similar results. Although reported in Italy in 2013 (1), to our knowledge, this is the first report of A. japonica on arugula in the United States.

References: (1) G. Gilardi et al. Acta Hort. 1005:569, 2013. (2) E. G. Simmons. Page 368 in: Alternaria, An Identification Manual. CBS Fungal Biodiversity Centre, Utrecht, 2007. (3) S. Werres et al. Z. Planzenkr. Pflanzensh. 108:113, 2001. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.