Crown Rust

Puccinia coronata is a plant pathogen and causal agent of oat and barley crown rust. The pathogen occurs worldwide, infecting both wild and cultivated oats. Crown rust poses a threat to barley production, because the first infections in barley occur early in the season from local inoculum. In oats, damage fluctuates per year, and may be trivial in damage some years or may account for over 30% of crop damage other years. Crown rusts have evolved many different physiological races within different species in response to host resistance. Each pathogenic race can attack a specific line of plants within the species typical host. For example, there are over 290 races of P. coronata. Crops with resistant phenotypes are often released, but within a few years virulent races have arisen and P. coronata can infect them..

Excerpt from the Wikipedia article Puccinia coronata, by its authors, under the CC BY-SA 4.0 licence.