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Etiology of Flower Necrosis in Cattleya Orchids. R. H. Lawson, Pathologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 60:36-40. Accepted for publication 11 July 1969. DOI: 10.1094/Phyto-60-36.

Rod-shaped virus particles were partially purified from a Cattleya orchid plant showing symptoms of severe flower necrosis. Differential and rate-zonal centrifugation separated three ultraviolet absorbing peaks. Fraction a at the top of the gradient column contained small particulate contaminants, fragments of virus-like rod particles, and a few longer rodshaped particles. Fraction b contained a high concentration of rod particles. Seventy-four per cent of the particles in this fraction measured 415-445 mµ. Fraction c contained a low concentration of particles 450-475 mµ long. Fractions a and c produced only a few lesions on Cassia occidentalis, but fraction b produced many local lesions on this test plant. Cymbidium mosaic virus antiserum was incubated with each of the three gradient fractions. No unreacted particles or particle fragments were observed in electron microscope examination of antiserum-absorbed fractions. Cattleya seedlings from Bow Bells crosses inoculated with gradient fraction b incubated with normal serum developed flower necrosis. Fraction b absorbed with CyMV antiserum produced no flower necrosis, but flower necrosis was produced by fraction b absorbed only with TMV-O antiserum. Although the orchid strain of tobacco mosaic virus is present in some naturally infected plants with flower necrosis, this virus is not an essential component in the disease reaction.