First Report of White Rust of Gerbera Caused by Albugo tragopogonis in North America.

Abstract

In October, 1995, in Villa Guerrero, Mexico, white erumpent sori were detected on the undersurfaces of leaves of a bed of African daisy (Gerbera jamesonii H. Bolus ex J. D. Hook.). The white sori on the undersurfaces of leaves corresponded to chlorotic spots on the upper leaf surfaces. All gerbera plants examined were severely diseased. The gerbera planting was adjacent to a planting of chrysanthemums with symptoms of white rust caused by Puccinia horiana Henn. Symptoms on the leaves of the two crops looked similar, and therefore P. horiana was suspected of causing both diseases. However, when subepidermal sori (pustules) on Gerbera plants were examined microscopically, erumpent chains of round to cylindrical, hyaline to pale yellow sporangiospores borne on short club-shaped sporangiophores were found. The sporangiospores are typical of Albugo tragopogonis (Pers.) S. F. Gray, and it was concluded that the white rust found on gerbera in Villa Guerrero, Mexico, was caused by A. tragopogonis. This disease was reported on gerbera from Australia and New Zealand in 1965 (1) and from Spain in 1977 (2). Other hosts of A. tragopogonis include members in the genera Pericallis (cineraria), Centaurea (dusty miller, basket flower, cornflower, bachelor's-buttons), Ambrosia (common ragweed), Tragopogon (salsify), Antennaria (everlasting, pussytoes), Artemisia (wormwort, mugwort), Cirsium (thistle), Matricaria (false chamomile, pineapple weed), Iva (marshelder), Parthenium (guayule, American feverfew), and Xanthium (common cocklebur, spiny cocklebur). This is the first report of white rust of gerbera caused by A. tragopogonis in North America, and may represent a new disease problem for species of important floral crops in the future.