Common groundsel (Senecio vulgaris, Asteraceae) is native to Europe and is now a common weed mainly in disturbed habitats of almost worldwide distribution. In November 2000, groundsel plants growing adjacent to lettuce fields in California's coastal Salinas Valley (Monterey County) showed symptoms of rust. In a 0.2-ha survey area, 75% of the plants were infected. Examination of weeds growing in four residential blocks also uncovered infected groundsel. Densely clustered, orange aecia were observed on leaves and stems. Stems were swollen where aecia had formed. Blossom and fruit formation was not notably reduced, although some involucral bracts were infected. Aeciospores measured 14 to 18 μm × 12.5 to 15 μm (fresh material). Telia were not found. The pathogen was identified as Puccinia lagenophorae Cooke, a rust fungus that is native to Australia and New Zealand and infects plants of the subfamily Asteroideae (family Asteraceae) (3). P. lagenophorae is an autoecious species forming only repeating aecia (stage I) and telia (stage III). There are six other rusts of Senecio that occur in the United States (1) that readily form aecia but not telia on Senecio spp. When only aecia are observed on Senecio, which is typical for P. lagenophorae (2), the following features can be used to differentiate it from these other species: no pycnia (stage 0) are formed; aecia are formed repeatedly; systemic growth that results in deformation of the host, including formation of galls with dense clusters of aecia on the stem; poorly developed aecial peridium; and aeciospores small, measuring 12.5 to 18.5 μm × 10.0 to 16.0 μm (4). In addition, P. lagenophorae forms aecia even at the end of the year in northern temperate zones, whereas heteroecious species form aecia only in spring and early summer (2). This is the first record of P. lagenophorae in North America. Specimens were deposited in the Arthur Herbarium, Purdue University. Outside its native habitat, this fungus has been found in Europe, Africa, the Middle East, and South America. There are about 60 known host species of P. lagenophorae (3) including ornamentals such as Bellis perennis, Calendula officinalis, and Senecio cruentus. The pathogen may have been introduced to North America via land from South America through Central America, or by the importation of ornamentals that were either infected with rust or infested with diseased groundsel.
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) M. Scholler. Regensb. Myk. Schr. 6:1, 1996. (3) M. Scholler. J. Plant Dis. Prot. 105:239, 1998. (4) I. Wilson et al. Trans. Brit. Mycol. Soc. 48:501, 1965.