A basic knowledge of the distribution and incidence of a pathogen throughout its hosts’ range is fundamental to any ecological or evolutionary study of disease. In the case of two annual sunflower species (Helianthus annuus and Helianthus petiolaris), the fungal pathogen Puccinia helianthi is known to have a broad geographical distribution. However, there is little information on the relative incidence of the pathogen across these hosts’ ranges. Although H. annuus occurs worldwide while H. petiolaris has a more limited range, the two species co-occur within the central region of North America, from southern Canada to northern Mexico. One might hypothesize that longer growing seasons in the southern range may lead to more generations of rust, and therefore a higher incidence of the disease. However, past work by sunflower pathologists provides evidence that resistance to rust is more common in sunflower populations with longer growing periods. The goal of this study was to test the above hypotheses. It is, however, difficult to quantify the incidence of a plant disease over large geographic areas. Therefore, we scored herbarium specimens of H. annuus and H. petiolaris for the presence/absence of disease, and recorded collection date, geographic location and leaf area.

Results/Conclusions

We found significant effects of species, collection date, and the interaction of species and geographic location on rust incidence. Overall, rust was more common on H. annuus than on H. petiolaris. There was significantly lower rust incidence in Kansas than in Texas for H. petiolaris, which supports the first hypothesis. In contrast, there was significantly higher rust incidence in H. annuus in Kansas than in Texas, supporting the second hypothesis. As might be expected for a fungus that goes through multiple generations in each host growing season, the percentage of plants of both species infected increased with collection date. In the future, herbarium-based studies of pathogen distribution combined with ecological studies and genetic knowledge of host resistance and virulence patterns can contribute to an understanding of the interactions between plant species and their pathogens.