Black cherry trees, native to the United States, are an invasive species in Europe and thrive in that part of the world. Experiments show why: A soil-borne pathogen keeps these trees in check in the United States, but is too weak to stop them from spreading in Europe.

That's according to a study by Agricultural Research Service (ARS) ecologist Kurt Reinhart at the agency's Fort Keogh Livestock and Range Research Laboratory in Miles City, Mont. He and cooperators collected soil randomly around black cherry trees in more than 20 forests throughout their range in the United States, and nearly 20 forests throughout their range in Germany, France, Belgium and The Netherlands. They isolated the pathogen, called Pythium, from the soil samples.

Pythium "damping-off disease" kills seedlings in farm fields and greenhouses as well as trees in forests.

Reinhart and colleagues tested the virulence of each Pythium isolate. They then used DNA sequencing to identify each isolate. They found that some nonaggressive Pythium types were common in both ranges, but aggressive types were found only among samples from the tree's native range.

The study is unique because the scientists tested the virulence of soil-borne pathogens associated with cherry trees in the trees' native and non-native ranges. Other studies have documented variation in the number of pathogen species associated with plants in their native versus non-native ranges, but have not determined virulence.

Demographic research by the scientists indicates that black cherry trees grow much more sparsely in native than in European forests. This pattern, coupled with results from the pathogenicity experiments, suggests that Pythium helps regulate black cherry populations in the United States, but not in European forests.

Evidence of an invader encountering more aggressive enemies in its native versus non-native range provides new evidence for the popular hypothesis that invasive species-whether plants, insects, or other animals-thrive outside of their native lands in part because they have escaped their enemies.

Reinhart will summarize results from this study at the Soil Ecology Society and Society of Nematologists Joint Meeting this week in Burlington, Vt.