Puccinia areolata, P. treleasiana, and P. gemella on Marshmarigold ( Caltha spp.) in subalpine habitats in Northwestern United States.

Marshmarigolds ( Caltha spp.) are members of the buttercup family (Ranunculaceae) and inhabit the Cascade and Rocky Mountains in western North America. Several rust species infect Caltha species, but only Puccinia areolata, Puccinia treleasiana , and Puccinia gemella have been reported in the Pacific Northwest. Observations were made on rust incidence and severity on Caltha spp. in July through September between 2001 and 2013 in subalpine regions of Colorado, Idaho, Montana, Utah, Washington and Wyoming. Leaves, petioles and flower stalks with rust pustules were examined and the host species identified using morphological characteristics. Plants collected in ID, WA, and MT were identified as Caltha leptosepala ssp. howelli (syn Caltha biflora ) and plants in WY, CO, and UT were identified as Caltha leptosepala ssp. leptosepala . Foci of infection were typically solitary leaves, individual plants or small clusters of infected plants although a 90% incidence of infected rust plants was observed in a 10 m diameter area with no visually distinct foci in WA in 2013. Puccinia areolata was the only rust observed in Washington, Puccinia gemella was observed in Idaho, and Puccinia treleasiana was observed in Montana and Wyoming. Collections made possible direct comparisons among the three rust species.