Rust on Colt's-foot

Pucciniaa poarum on Tussilago 21-6-2013
(c) N. Stringer

In response to a query regarding a visit to Craig-y-Llyn by Ian Morgan and Nigel Stringer, Nigel wrote: The rust on Colt's-foot (Tussilago farfara) is conspicuous at this time of year. Colt's-foot is characteristic of weedy, bare ground, but we found it at Craig y Llyn in the fantastic alpine wet flushes. Only one leaf was infected with one rust pustule at the latter,  but the Colt's-foot growing on the verge of the track from the main road to the lake was heavily infected. This difference in the degree of infection between the two habitats is because the 'alternate' host of the rust (Puccinia poarum) is a grass belonging to the genus Poa. The most common Poa is Poa trivialis (Rough Meadow-grass) which becomes infected in summer & autumn.

Hence the (indirect evidence due to degree of infection of Colt's-foot)  probability that there is more P. trivialis growing along the trackside than up in the alpine flush. The single infection of one leaf in the latter habitat is very probably due to a chance spore being blown from a plant growing along the trackside and being deposited on a Colt's-foot leaf.

Rust spores (depending on the species) can travel thousands of miles (e.g.  infection of the North American cereal crops by spores originating off plants growing in Mexico).