Spatial connectedness of plant species: potential links for apparent competition via plant diseases

This study evaluated the reactions of seven common C4 grasses of the tallgrass prairie of the USA Great Plains to the economically important wheat pathogens Pyrenophora tritici-repentis and Gaeumannomyces graminis var. tritici (Ggt) isolated from wheat. The P. tritici-repentis isolates (race 1) were pathogenic on all grasses tested, but symptom severity was markedly low. Three of the grass species inoculated with Ggt were highly susceptible, while four species exhibited no symptoms. Because measures of connectedness can provide a proxy for population processes, connectedness was evaluated within and among the seven grass species in representative tallgrass prairie environments for all potential pathogen-sharing patterns. Andropogon gerardii was ubiquitous, so all plant species were well connected to it. Andropogon scoparius (= Schizachyrium scoparium), Sorghastrum nutans and Panicum virgatum were fairly common but specialized to particular environments. Bouteloua curtipendula was uncommon but occurred in all environments, while Buchlo€e dactyloides and Bouteloua gracilis were uncommon and only occurred in upland sites. Co-occurrence of plant species was generally not reciprocal in that, for many species pairs, species A rarely occurred without potential exposure to inoculum from species B, while species B commonly occurred without species A. The three grass species susceptible to Ggt may act as sources of inoculum for each other within tallgrass prairie, with the potential to influence fitness, and tallgrass prairie and commercial wheat ecosystems in the Great Plains also have the potential to share both pathogens.