Spatial and temporal dynamics of Puccinia andropogonis on Comandra umbellata and Andropogon gerardii in a native prairie

The rust Puccinia andropogonis (Schwein.) was studied on its aecial and telial plant hosts, comandra (Comandra umbellata L. Nutt.) and big bluestem (Andropogon gerardii Vitman), respectively, in a native prairie to investigate the influence of aecial host proximity on disease severity of the telial host in a natural system. Both hosts were mapped to measure distances from comandra clones to selected big bluestem plants in a Minnesota prairie. Mean rust severity on big bluestem was regressed on the number of aecia on comandra (aecial density) within eight distance intervals from big bluestem plants. Distance intervals were analyzed both inclusive and exclusive of other distance intervals. There was a significant positive relationship between aecial density on comandra and rust severity on big bluestem that decreased with increasing distance in accordance with the power law model, becoming nonsignificant at distances >40 m. To establish whether the genetic background of big bluestem influenced the relationship between rust severity and the distance to comandra, a common garden was planted with a representative big bluestem plant population. The low coefficient of determination (r²) between mean rust severities of individual plants from one year to another suggests there is not a strong genetic component in the host determining disease severity.