Cosmos (Cosmos bipinnatus Cav.) is an annual that is grown for cut flowers or as a landscape bedding plant. In late July 2009, cosmos plants were collected from a 0.4-ha field in Santa Barbara County, CA and submitted to the California Department of Food and Agriculture's Plant Pest Diagnostics Laboratory. Plants showed symptoms of chlorosis, wilting, necrosis, and death. Symptomatic plants comprised approximately 50% of the crop. Roots and stems appeared entirely discolored. Pieces (4 mm3) were taken from the edges of the discolored tissue of roots and stems, surface sterilized in 0.6% NaOCl for 2 min, and placed onto one-half-strength acidified potato dextrose agar (APDA). Fungal colonies consisted of fine, hyaline hyphae with verticillate conidiophores producing hyaline conidia, measuring 4.2 to 7.0 × 1.8 to 3.0 μm (5.13 × 2.44 μm average), in slimy masses. Microsclerotia (30.0 to 137.5 × 15.0 to 60.0 μm, 57.6 × 33.7 μm average) formed after 1 week in culture, causing the center of the colony to darken. Morphological characteristics were consistent with those of Verticillium dahliae (2). The internal transcribed spacer region (ITS) of rDNA was amplified for one isolate from cosmos using ITS1 and ITS4 primers as described by White et al. (3), and the amplicon was sequenced (GenBank Accession No. GU99602). BLAST analysis of the 455-bp amplicon showed 100% identity with the ITS sequence of V. dahliae from cosmos in Italy (GenBank Accession No. GQ130129). Pathogenicity of the California cosmos isolate of V. dahliae was determined by inoculating 10 1-month-old seedlings (each approximately 20 cm high) of C. bipinnatus ‘Sensation Mix’ with this isolate. Plants were inoculated with spores harvested by flooding 2-week-old cultures of V. dahliae grown on APDA medium with sterile distilled water. Plant root tips were trimmed and submerged in a spore suspension (1.3 × 106 spores/ml) for 5 min. Ten plants were dipped in water as the negative control treatment. Plants were then planted in a commercial potting mix in 10-cm-diameter pots and kept in a growth chamber at 25°C with a 12-h photoperiod. Inoculated plants were chlorotic and wilted when root and stem isolations were performed 1 month after inoculation. V. dahliae grew from stems and roots of 7 and 2 of the 10 inoculated plants, respectively. The inoculation experiment was repeated on six 5-month-old plants with similar results, except V. dahliae was isolated from the roots and stems of six and five plants, respectively. No symptoms developed on plants dipped in water, and Verticillium spp. did not grow from isolated root or stem pieces from the noninoculated plants in either experiment. On the basis of morphological and ITS sequence information, the fungus was identified as V. dahliae. V. dahliae is an economically important pathogen with a wide host range worldwide including cosmos in Italy (1). The affected field in California had a history of vegetable and flower seed crops, as well as vegetables for consumption. Infection of cosmos may have been from soilborne microsclerotia from previous susceptible crops. To our knowledge, this is the first report of Verticillium wilt on cosmos in California.
References: (1) A. Carlucci et al. Plant Dis. 93:846, 2009. (2) D. L. Hawksworth and P. W. Talboys. No. 256 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1970. (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.