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First Report of the Rust Miyagia pseudosphaeria on Sonchus oleraceus in the Americas

In January 2002, a rust was observed on leaves and stems of the weed Sonchus oleraceus L. (annual sowthistle) in Santa Barbara, CA. Infected plants were collected in Santa Barbara and Monterey counties, CA. Specimens were examined microscopically and compared with a published description (1) and herbarium specimens at the USDA National Fungus Collection, Beltsville, Maryland. The fungus was identified as Miyagia pseudosphaeria (Mont.) Jorst. based on characteristics of the uredinia and urediniospores. The uredinial sori were subepidermal, erumpent, and single or in groups. Sori were surrounded by peripheral, laterally connected, chestnut brown paraphyses that initially enclosed the developing urediniospores and eventually opened partially at the apex, breaking the host epidermis and remaining as a wall surrounding the spores. The urediniospores were ellipsoid, ovoid, or pyriform with yellowish, often granular, contents and measured 21.5 to 43 × 17 to 26.5 μm. The urediniospore walls were hyaline and finely echinulate and 2.5 to 3.5 μm thick with germ pores sometimes evident in median view. The paraphyses were 70 to 140 × 6 to 8 μm, thick-walled, hyaline below, chestnut brown above, and often somewhat wider or dichotomously branched at the apex. To test pathogenicity, a spore suspension (4.7 × 10⁴ spores/ml) was sprayed onto 7-week-old annual sowthistle plants. Plants were incubated in a dew chamber for 48 h and maintained in a greenhouse at 22 to 24°C. Signs of rust were observed after 2 weeks, and microscopic examination confirmed that the same fungus was present. Uninoculated control plants did not develop rust. Using the same method, plants of prickly sowthistle (S. asper (L.) Hill) were inoculated and also developed the disease. All inoculation experiments were conducted two times and the results were the same. The rust was found on almost all annual sowthistle plants examined in Santa Barbara County, especially well-nourished specimens growing as weeds in irrigated landscapes, vegetable fields, or in container nurseries. Diseased plants were found in and around vegetable fields in Monterey County. M. pseudosphaeria is macrocyclic, autoecious, and known only on Sonchus spp. It has been reported from northern Africa, China, Europe, Japan, New Zealand, and the former Soviet Union. To our knowledge, this is the first report of M. pseudosphaeria in the Americas. Voucher specimens have been deposited in the U.S. National Fungus Collection (BPI 842025, 842055 to 842060, and 842230). Images can be viewed at nt.ars-grin.gov.

References: (1) M. Wilson and D. M. Henderson. British Rust Fungi. Cambridge University Press, Cambridge, 1966.