Puccinia canaliculata (Schwein.) Lagerh. is a macrocyclic, heteroecious rust found on Cyperus spp. (sedges) in North and South America, with the aecial stage reported on Asteraceae (1). In July 1997, aecial pustules of unknown etiology were observed on lower leaves of more than 90% of cultivated sunflower (Helianthus annuus L.) plants at the 6- to 10-leaf stage in a field near Catherine, KS. Individual leaves had one to several large, convex pustules, each measuring 5 to 10 mm in diameter, covering <0.5% of the leaf area. In contrast, the aecial pustules of P. helianthi measure 1 to 2 mm in diameter. Yellow-orange aecial cups occurred on the abaxial leaf surface, with globoid aeciospores averaging 15 × 18.5 μm. Wild common sunflowers (H. annuus) and cocklebur (Xanthium strumarium L.) in the same and two nearby fields had similar pustules. There was a severe infestation of yellow nutsedge (C. esculentus L.) in the sunflower field, but uredia were not found because the nutsedge plants had been killed with glyphosate. Based on the size of aecia, aeciospores, and peridial cells from sunflower and cockelbur (3), the fungus was identified as P. canaliculata. Although sunflower is an alternate host (2), to our knowledge this is the first report of natural infection and is significant because it occurred in a major sunflower production area. Since P. canaliculata is being considered as a bioherbicide for nutsedge control (4), nontarget hosts such as sunflower need to be considered. The possibility of confusing P. xanthii and P. caniculata exists since both rusts occur on cocklebur and sunflower and produce pustules of a similar size. However, since P. xanthii is a microcyclic autoecious rust while P. canaliculata is a full-cycle heteroecious rust, the obvious color difference between the dark telia of P. xanthii and the yellow-orange aecia of P. caniculata should serve to easily differentiate the two species.
References: (1) J. C. Arthur. Manual of the Rusts in the United States and Canada. Purdue Research Foundation, Lafayette, IN, 1934. (2) M. B. Callaway et al. Plant Dis. 69:924, 1985. (3) F. D. Kern. Mycologia 11:134, 1919. (4) S. C. Phatak et al. Science. 219:1446, 1983.
