Oct. 8, 1999 BLOOMINGTON, Ind. -- A microscopic fungus that lives inside the most common kind of grass in the eastern United States may be reducing plant diversity throughout its expanding range. The fungus provides competitive advantages for its host, tall fescue, but it is toxic to livestock, wildlife and insects that eat the infected grass. The economic consequences may be as much as a billion dollars a year in the livestock industry alone, and there are other consequences in both agriculture and conservation.
In a report in the Sept. 10 issue of the journal Science, Indiana University biologists Keith Clay and Jenny Holah describe the results of a four-year experiment with tall fescue at the IU Botany Experimental Field. Their study demonstrates that an internal fungus called an endophyte, Neotyphodium coenophialum, often found in tall fescue, can have a major effect on the number of other species persisting in areas where the infected grass grows.
"The infected plants are more vigorous, more toxic to herbivores, and more drought-resistant than uninfected plants," Professor Clay said. "Tall fescue was introduced from Europe in the last century, and it has been widely planted for forage, turf and soil conservation. But it is also a tenacious invader of natural communities, where it can displace native plant species and reduce wildlife populations. Fescue is now the most abundant perennial grass in the eastern United States, and about two-thirds of it is infected with the fungus, which also is not native to North America. The potential effect of the fungus on communities is large, given the abundance and distribution of its host."
Infected grass can't be identified by sight, because the leaves look the same as uninfected grass. The fungus lives in the spaces between the plant's cells, and there is no external sign of its presence. The fungus infects only fescue, because it spreads only within seeds of infected plants. So the fungus is not transmitted to other plants, and only infected fescue reaps the considerable benefits of the fungus.
Clay and Holah established eight experimental plots in summer 1994, each of which had a variety of vegetation at the start. Alternating plots were then planted with infected or uninfected fescue seed. After two months, there were stands of fescue in each plot along with many other species. No other treatments were used, such as fertilizer, pesticides or mowing.
After four years, infected plots had become dominated by fescue -- nearly 90 percent of the biomass -- while uninfected plots had more diverse populations of other grasses such as Kentucky bluegrass, with fescue making up less than 60 percent of the biomass.
The total biomass did not differ between infected and uninfected plots, so net productivity was not affected. The fungus could affect the community by several other methods, however.
"The fungus produces toxic alkaloids, which may have altered the feeding patterns of small mammalian herbivores, birds and insects that were abundant in our plots," Clay said. "In addition, infection by the fungus enhances drought tolerance of fescue, and there are often extended periods without rain during summers in southern Indiana. Finally, infected plants are more productive than uninfected plants in the absence of stress. Any or all of these factors may promote the dominance of infected fescue in mixed vegetation."
The results of the study have implications for conservation as well as agriculture, he added, because loss of plant diversity is likely wherever fescue is common and highly infected, not just in areas where it was planted.
"Independent evidence suggests that infection increases in mixtures of plant species over time, so that sites with low infection can become highly infected," he explained. "Seed transmission of the fungus ensures that it is dispersed with the grass. Given its widespread distribution and high infection frequency, tall fescue may threaten the persistence of native plant species and associated animals, affect succession, and modify food webs in plant communities."
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