Andropogon virginicus is a C4 perennial grass dominating successional old fields in its native range in eastern North America. It is naturalizing on hydric soils in California, and is invasive in the Hawai’ian islands. In many eastern populations this grass is attacked by the smut fungus Sporisorium ellisii, which replaces the plant’s reproductive structures with its own teliospores. If this pathogen influences its host’s individual and population growth, then escape from the disease may promote A. virginicus invasion in the introduced range, as suggested by the enemy release hypothesis and the evolution of increased competitive ability hypothesis. Infected native populations were censused annually from 2003-2006, with measures of host and disease density in 1m2 plots, and mortality, size, infection status, and photosynthesis rates in focal plants. Multiple populations in the eastern U.S., California, and Hawai’i were sampled in 1 m belt transects for plant density and disease presence, and up to 80 collected plants per population were measured for height and mass, and examined for smut fungus. The populations were compared in two common-greenhouse experiments. One measured seed germination percentages and time to germination, and the other measured photosynthesis rates and competitive ability against a phytometer.
In the main native population, mean plant density fluctuated across years, from 3.26-5.47 plants/m2, while mean disease frequency declined, from 29.8% to 5.9% infected plants/m2. In plots with disease present, disease was negatively correlated with plant density (2004: r = -0.50, 2005: -0.45, 2006: -0.63; all P < 0.001, with correction for spatial autocorrelation). Fully infected plants were smaller than healthy plants and produced no seeds. Although 21.2% of infected plants recovered, 70% died after two years, compared to 43% of healthy plants. Eastern populations averaged 2.96 plants/m2. Out of 41 populations, 22 were infected.