First Report of White Rust Caused by Albugo tragopogonis on Sunflower in Belgium.

Abstract

Sunflower (Helianthus annuus) is widely used for cut flowers and decoration in Belgium. A serious outbreak of what was suspected to be white rust on sunflower was observed in an East Flemish nursery near the city of Ghent in August 2004. This disease has previously been reported in Europe (southwest of France) (1) and other parts of the world with losses as much as 70 to 80% (Australia, North and South America, and Africa) (2,3). In the Flemish nursery, only single diseased plants (cv. Sunrich) were found. Blister-like pustules containing sporangia were observed on infected leaves. Initially the blisters were pale green to yellow on the abaxial surface and white on the adaxial surface of the leaves. As the disease progressed, white pustules that formed on the adaxial surface of the leaves slowly turned yellow, and the blisters on the abaxial surface became yellow to orange and necrotic in the center. Finally, the pustules coalesced and the leaves withered. Stem lesions were not observed. Short, cylindrical to spherical-cuboid sporangia, recovered from the pustules on the adaxial surface of leaves, measured between 17.5 and 22.5 μm, with an average of 20.2 μm. Sporangial dimensions were similar to those of Albugo tragopogonis (Pers.) S.F. Gray (1). Inoculations were done by spraying a suspension of 1 × 105 sporangia per ml prepared by scraping pustules from naturally infected leaves. Leaves on three 2-month-old healthy plants were sprayed with this inoculum and three plants sprayed with distilled water served as controls. The plants were kept for 48 h under a humid chamber and subsequently at room temperature (20 to 25°C) on the laboratory bench. Initial symptoms of white rust were observed 12 to 14 days after inoculation. On the basis of symptoms, morphological characteristics, and pathogenicity tests, the pathogen was identified as A. tragopogonis. To our knowledge, this is the first record of A. tragopogonis on H. annuus in Belgium.