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First Report of Ramularia carthami, Causal Agent of Ramularia Leaf Spot of Safflower, in California

Ramularia leaf spot was identified in several fields of safflower (Carthamus tinctorius) near Gridley, CA in June 2005. Numerous circular to irregularly shaped brown lesions, 3 to 10 mm in diameter, on both sides of leaves and flower bracts resulted in stunted plants and reduced seed production. In two of the fields, nearly all plants were affected, yields were severely reduced, and the crops were abandoned. Ramularia carthami Zaprom. was identified on the basis of morphology of reproductive structures on colonized leaves (1). Hyaline, thin-walled, aseptate conidiophores (2.6 to 4.3 × 28.8 to 72.0 μm) were produced in fan-like fascicles borne on hemispherical stromata (21.6 to 31.2 × 24.0 to 36.0 μm). Hyaline, smooth, cylindrical to fusiform conidia (7.2 to 12.0 × 19.2 to 40.8 μm), 1 to 3 septate or rarely aseptate were produced singly or in short chains. The fungus was isolated from symptomatic leaves and bracts surface disinfected for 1 min in 0.5% sodium hypochlorite and incubated at 25°C on acidified potato dextrose agar (APDA). Colonies of the fungus were white with irregular margins and were slow growing. After 3 weeks, colonies were approximately 3 cm in diameter. Conidia were not produced in culture. To conduct pathogenicity tests, three 3-week-old safflower plants grown in the greenhouse were sprayed with an aqueous suspension of mycelial fragments of the fungus. Inoculum was produced by macerating a 3-cm-diameter APDA culture of the fungus in 30 ml of water. Noninoculated control plants were sprayed with water. All plants were covered with plastic bags for 48 h on a greenhouse bench. Greenhouse temperatures ranged from a minimum of 20°C to a maximum of 27°C. After 7 days, all inoculated plants developed symptoms, and the fungus was reisolated from lesions. Conidia from lesions were suspended in water and diluted to a concentration of 1 × 10⁵ conidia/ml and used as inoculum for additional pathogenicity tests. Three plants were sprayed with the conidial suspension or water as above. Lesions developed on the inoculated plants in 7 days, and the fungus was reisolated. No symptoms developed on plants sprayed with water. Both pathogenicity tests were repeated once. Sequence of the internal transcribed spacer region of rDNA of the fungus was deposited in GenBank (Accession No. DQ466083). To our knowledge, this is the first confirmed report of Ramularia leaf spot of safflower caused by R. carthami in California.

Reference: (1) Morbi Plant. Script. Sect. Phytopath. Hort. Bot. Prince. USSR 15:142, 1926.