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Etiology

Pathogenicity and Conditions for Infection of Chrysanthemum and Rose Flowers by Bipolaris (Helminthosporium) setariae. Arthur W. Engelhard, Agricultural Research and Education Center, IFAS, University of Florida, 5007 60th Street East, Bradenton 33505; Phytopathology 66:389-391. Accepted for publication 14 October 1975. DOI: 10.1094/Phyto-66-389.

Inoculation with Bipolaris (Helminthosporium) setariae induced lesions on the flowers of chrysanthemum cultivars Doubloon, Southern Comfort, Tinsel, Yellow Knight, and Yellow Shasta but not on those of Blue Chip, Bronze Chip, Dolly, Iceberg, Yellow Iceberg, or Jackstraw. Flowers of the hybrid tea rose cultivars Improved Red American Beauty and Tropicana also were very susceptible. Reddish brown lesions, two to three times as long as broad and 1-2 mm in length developed on chrysanthemums. The lesions did not further increase in size. On roses, tan-colored spots up to 2 mm in diameter initially developed. With time, the spots increased in size and coalesced, resulting in large, tan, necrotic areas on the petals. Severely infected petals fell from the flowers. Severe disease developed at 29 and 24 C, with increasingly lower levels at 18, 13, 35, and 7 C, respectively, Seventeen percent of the petal tissue was necrotic (percent disease) on inoculated rose flowers covered with polyethylene bags for 3 hours (high humidity and free water on the petals) at 25 ± 1 C. Disease became progressively more severe as the duration of the incubation period in polyethylene bags increased to 6, 12, 24, and 30 hours. Sixty-two percent disease was present after 30 hours.