RESUMO
White rust, caused by Albugo tragopogonis (Pers.) S.F. Gray, was observed on a few plants of both oilseed and confection sunflowers (Helianthus annuus L.) in northwestern Kansas (Cheyenne County) in 1992. The disease was observed again from 1993 to1995 in nine counties in western Kansas, with incidence per field ranging up to 35%. White rust was found only on late-planted fields in 1996 and 1997 and was not found at all from 1998 to 2001. White rust was also observed on cultivated and wild sunflower (H. annuus) for the first time in eastern Colorado (Kit Carson and Yuma counties) from 1994 to 1997, but was absent from 1998 to 2001. Leaf pustules on both cultivated and wild sunflowers were similar in appearance. Pustules were convex, chlorotic on the upper side of the leaf, and concave and dull white on the under side of the leaf. Pustules on cultivated sunflower were generally limited to three to six leaves in the middle of the plant and affected 10 to 40% of the leaf area. Sporangial dimensions fell within the reported dimensions for A. tragopogonis (2). In 1997, water-soaked lesions 1 to 2 cm long containing oospores of A. tragopogonia were observed on the lower to middle portions of stems of cultivated sunflower in western Kansas and the adjacent area of Colorado. Stem lesions were observed much less frequently than foliar lesions and only in 1997. Sporangia were not observed in stem lesions, nor were any other fungi isolated from these lesions. To our knowledge, this is the first report of white rust occurring on cultivated sunflower in any production area of North America; the disease has not been observed in the major U.S. sunflower production area of North Dakota, South Dakota, and Minnesota. Foliar white rust lesions generally have little economic impact on sunflower, but the presence of stem lesions is significant because stem lesions may lead to lodging (3). Lodging due to A. tragopogonia was not observed in either Kansas or Colorado. White rust has previously only been reported on wild H. annuus in Wisconsin and on perennial Helianthus spp. in Missouri and Illinois (1). References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN 1989. (2) K. G. Mukeri. Description of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK 1976. (2) P. S. van Wyk et al. Helia 22:83, 1995.