During a period of wet weather from December 1996 to March 1997, commercial plantings of sunflower in San Diego County, CA, were infected by a leaf smut, Entyloma polysporum (Peck) Farl. The fungus was observed on sunflowers grown in a greenhouse in San Diego County and also on sunflowers from nurseries in Ventura and Riverside counties. Although the disease was first noticed in 1996, the infection was not of economic significance so no attempt was made to identify the causal agent. However, with continuous cropping of sunflowers year round significant losses were observed on seedlings that were systemically infected as they emerged. This is the first report of E. polysporum causing economic losses on sunflowers. The distinguishing characteristics of this fungus are masses of globose to subglobose spores, pale green to yellow green in color, approximately 12 μm in diameter, with a double wall consisting of an inner pale green wall and outer hyaline sheath. The spores occur in dense masses called sori that completely replace the leaf cells. Young spores are difficult to distinguish from leaf cells in a cursory examination. Older sori form discolored lesions in the leaf ranging from circular to irregular in shape and replace most of the chlorenchyma tissue in the infected lesions. Identification of species of smut fungi such as Entyloma is based on the location of the sori in the vegetative parts of the host, the identification of the host, and the spore morphology (4). Savile (3) reviewed the genus on North American composites and, based on morphological characteristics, concluded that almost all the pathogens were either E. compositarum or E. polysporum, with a few intermediate forms. E. polysporum is characterized by globose spores 10 to 17 μm in diameter, surrounded by cell walls 1 μm thick encased in a 1.5- to 2.5-μm thick smooth hyaline sheath (1). Spores of E. compositarum are smaller, 9 to 12 μm in diameter, thin walled (1 to 1.5 μm), smooth, and without a sheath (2). Vánky (4) lists 33 different species on composites according to their host. He believes E. polysporum only occurs on Ambrosia spp., and does not include E. compositarum in his list of Entyloma spp. Neither author mentions Entyloma infecting any species of Helianthus. Savile concluded that E. calendulae (Oudem.) de Bary, described in Europe, is very similar morphologically to E. polysporum, and is probably the same species. E. polysporum was first described in 1881 by Peck as Protomyces polysporus infecting Ambrosia trifida in New York State. In 1996, it was described on Ambrosia artemesifolia in Hungary (3). In the United States it has been reported on sunflowers in Montana (1,2).
References: (1) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN. (2) G. W. Fischer. 1953. Manual of the North American Smut Fungi. Ronald Press, New York. (3) D. B. O. Savile. Can. J. Res. 25(C):109, 1947. (4) K. Vánky. 1994. European Smut Fungi. Gustav Fischer, New York.
