Cosmos (Cosmos bipinnatus Cav., Asteraceae) is an herbaceous plant that is grown for landscape use. During August and September of 2008 in five public and three private gardens located in Monopoli (Apulia, southern Italy), 3 to 8% of the plants showed severe symptoms of vine decline, stunting, gradual yellowing and wilting of the leaves, and final collapse of the whole plant. External symptoms were associated with brown or black streaking of the vascular tissue of roots, collar, and stem. Dead plants had numerous microsclerotia embedded in the xylem of plant tissues. Stem, collar, and root sections (0.5 cm long) from symptomatic plants collected in five gardens were surface disinfested in 5% NaOCl for 1 min and transferred to petri dishes containing potato dextrose agar (PDA) amended with 100 μg ml--1 of streptomycin sulfate and 10 μg ml--1 of neomycin. After 10 days of incubation, at 25°C in the dark, hyaline hyphae with dark microsclerotia (37 to 112 μm) and verticillate conidiophores were produced. Conidia were single celled and hyaline with dimensions of 3.3 to 7.8 × 1.8 to 3.3 μm (mean dimensions 4.2 × 2.5 μm). According to morphological characteristics, the fungus was identified as Verticillium dahliae Kleb. (1) (isolates no. Vd1818, Vd1819, and Vd1820 stored in a collection at the Department DiSACD, University of Foggia). Molecular analyses were performed on the basis of nucleotide sequences of the internal transcribed spacer region (ITS1-5.8S-ITS2) of ribosomal DNA. ITS sequences of this fungus, compared with sequences found in GenBank and attributed to V. dahliae (no. GQ130129, GQ130130, GQ130131), showed 98 to 99% sequence similarity. Healthy 40-day-old plants of C. bipinnatus (garden cosmos) cv. Sonata Pink Blusk and C. sulphurous (yellow cosmos) cv. Bilbo, obtained from seeds previously disinfested for 1 min in 3% NaOCl and ascertained to be healthy by isolation on PDA medium, were used for pathogenicity tests. Plants were grown in 3-liter pots in a steam-disinfested peat, sand, and soil mixture (2:1:1) in the greenhouse at 23 to 26°C. Ten plants of each cultivar were inoculated by root dipping into a conidial suspension of each fungal isolate (1.5 × 106 CFU ml--1). Six noninoculated cosmos plants of each cultivar served as controls. The experiment was repeated three times. First symptoms of wilting were observed on all inoculated plants of each cultivar 20 days after the inoculation; at 40 days, symptom severity ratings on plants were taken, in which 1 = asymptomatic, 2 = stunted, 3 = wilting, and 4 = dead. All three isolates caused vascular discoloration, stunting, wilting, and plant death. The mean disease rating was 3.2 and did not differ significantly among isolates. The pathogen was consistently reisolated from infected plants, fulfilling Koch's postulates. Noninoculated plants remained healthy. To our knowledge, this is the first report of Verticillium wilt on cosmos in Italy. The finding is important since other ornamental plants that are susceptible to Verticillium wilt are also grown in landscapes in the region. The disease was previously reported in Turkey (2).
References: (1) G. F. Pegg and B. L. Brandy. Verticillium Wilts. CABI Publishing, Wallingford, UK, 2002. (2) E. Sezgin et al. Turk. Phytopathol. 14:43, 1985.
