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