First Report of Alternaria Leaf Spot of Cilantro (Coriandrum sativum) Caused by Alternaria dauci in the United States

Cilantro (Coriandrum sativum), widely used in different types of cuisine worldwide, is an annual herb in the family Apiaceae, which contains over 3,000 species including parsley, celery, and carrot. In January 2017, small brown spots with a white center were observed on leaves of cilantro in a plantation in Homestead, FL. Approximately 15% of the plants in the field were infested. Small pieces (5 mm²) of the leaves containing both healthy and diseased tissues were excised, surface sterilized with 70% ethanol for 15 s, rinsed in sterilized water, followed by 5 min in 0.5% NaClO, and a final rinse in sterile water. The surface sterilized tissue was transferred to potato dextrose agar (PDA) plates and incubated at 37°C. In about 5 days, dark greenish fungal colonies were consistently grown from the tissue on PDA. The purified culture did not produce conidia on PDA, but it did on water agar after 1 week under a 24-h photoperiod at room temperature. The conidia were dark brown, obclavate with long tails, having 2 to 4 vertical and 2 to 7 transverse septa. The conidia were 8 to 10 ± 0.8 µm long and 2.5 to 4 ± 0.36 µm wide (n = 30), and the tails of the conidia were 25 to 35 ± 2.3 µm and 0.8 to 1 ± 0.06 µm wide (n = 30). Based on the morphological characteristics, the fungus was tentatively identified as Alternaria sp. (Barnett and Hunter 1998). To confirm the identity, the regions of two genes of a representative isolate FL-1, i.e., glyceraldehyde-3-phosphate dehydrogenase (gapdh), and Alternaria a1 major allergen (Alt a1) were amplified and sequenced with primers gpd1/gpd2 and Alt F/Alt R (Berbee et al. 1999), respectively. Sequences of the DNA regions were deposited to GenBank with accession nos. MF595071 (Alt a1) and MF595072 (gapdh). The BLAST results revealed that the causal organism was Alternaria dauci based on the fact that the sequences of the two genes had 100% identity only to A. dauci isolates at nucleotide level. Sequence of FL-1 isolate MF595071 (Alt a1) is 100% identical to GenBank accessions HE796725.1, HE796722.1, HE796721.1, and AY563292.1 of A. dauci isolates. The sequence of FL-1 isolate MF595072 (gapdh) is 100% identical to GenBank accessions HE796767.1 and HE796759.1 of A. dauci isolates. Pathogenicity test was performed by spraying a mixture of conidial/mycelium (1 × 10⁵ conidia+mycelia/ml) of FL-1 onto both surfaces of the leaves of 3-week-old cilantro plants. Ten plants were sprayed with conidial/mycelial inoculum and 10 were sprayed with deionized water as a control. All cilantro plants were maintained in a plastic chamber for 2 days. Blighted spots with a white center similar to the original cilantro plants developed on plants receiving conidial/mycelial inoculum 5 days after inoculation. The control plants did not exhibit any symptoms. The pathogenicity test was carried out twice and the fungus was reisolated from the cilantro plants. Similar mycelial growth was observed on PDA plates and the same fungal isolate was confirmed by RT-PCR. To our knowledge, this is the first report of A. dauci in cilantro in the United States. A. dauci has been reported in cilantro only in Mauritius (Orieux and Felix 1968) and Puerto Rico (Woudenberg et al. 2014). This new disease could be of high economic importance to growers in Florida and other regions of the U.S.A. where cilantro is commercially grown.