Puccinia xanthii Schwein., commonly known as cocklebur rust, is circumglobal on species of Xanthium and Ambrosia. This microcyclic rust has only been observed on oilseed sunflower (Helianthus annuus L.) in Australia (1) and on ornamental sunflowers in South Africa (4). In September 1999, large (4 to 10 mm), raised, chlorotic pustules were observed on the adaxial leaf surface of oilseed sunflower plants (Dekalb 3790) near Hettinger, ND. Telia were associated with the pustules on the abaxial leaf surface. No cocklebur (X. strumarium L.) plants were found in the field, but rust-infected cocklebur plants were collected several kilometers away. Approximately 10% of sunflower plants in the field were affected, and generally only one or two pustules were observed on one or two leaves per plant. In contrast, numerous leaves of cockleur plants were infected with 12 or more pustules. Teliospores from sunflower were brown, two-celled, and averaged 49 × 17 μm, with a distinctly thicker wall at the spore apex and a persistent pedicel averaging 40 μm long. Teliospores from cocklebur were morphologically similar to those from sunflower and averaged 46 × 16 μm. Size and morphology of teliospores from both hosts fit the description of P. xanthii (2). P. xanthii can be distinguished easily from the ubiquitous P. helianthi Schwein. because the latter has smaller telia (1 to 2 mm diameter) and produces wider teliospores (21 to 30 μm diameter). P. xanthii was not found in surveys of 20 other sunflower fields in southwestern North Dakota nor in 45 fields in eastern ND in 1999, nor was P. xanthii found in this or any other sunflower field in 2000 or 2001. To our knowledge, this is the first report of P. xanthii on cultivated or wild sunflower in North America. The relatively few pustules observed on oilseed sunflower agree with the observation that oilseed sunflowers are much less susceptible to P. xanthii (3) than Xanthium spp.
References: (1) J. L. Alcorn and J. K. Kochman. Austral. Plant Pathol. Soc. Newsl. 5:33, 1976. (2) G. B. Cummins. Rust Fungi on Legumes and Composites in North America. University of Arizona Press, Tucson, 1978. (3) J. B. Morin et al. Can. J. Bot. 71:959, 1993. (4) Z. A. Pretorius et al. Plant Dis. 84:924, 2000.