Wildflower is published quarterly by the Wildflower Center. Its content is national in scope with articles about the conservation and use of native plants as well as news from the Wildflower Center. A subscription is provided to Wildflower Center members as a benefit of membership.
Plant Pathogens by Sheryl Devore - Summer 2005
Something was happening to the American elm. This stately plant, the most beloved of urban trees in the United States, was dying. One by one, the leaves at the tops of many American elms, which lined U.S. urban streets, were wilting, curling, and turning yellow. By 1970, some 77 million elms had died nationwide, each having succumbed to an epidemic disease caused by a non-native fungus.
This troublesome fungus is a pathogen or a disease-causing agent. "A plant pathogen is a microorganism that derives its nutrition from a living plant," explains Larry Englander, a plant pathologist and teacher at the University of Rhode Island. Plant pathogens include fungi, bacteria, viruses, and nematodes, which are microscopic worm-like creatures.
Not all fungi, bacteria, and nematodes are pathogens - some even benefit the environment by recycling already-dead plants, thus helping with decomposition. Others benefit live plants by helping them obtain nutrients or fix nitrogen, according to Kerry Britton, national plant pathologist for the USDA Forest Service in Arlington, Virginia.
Plants have defenses that help them resist disease. One way they do this is to compartmentalize, says Ronald Billings, head of forest pest management for the Texas Forest Service. "A tree may be able to isolate the infected tissue and keep the disease from spreading," he explains.
Englander agrees, "Usually a balance is achieved between the native pathogens and their native hosts. [Outbreaks of] plant epidemics are (more often) the exception, not the rule. But when you pick up a pest or pathogen and move it to a new area where the plant population hasn't evolved to tolerate it, that's when you have problems."
That's what happened with the American elm. Elms native to China have evolved to resist a fungus native to China (Ophiostoma ulmi) that preys upon them. That fungus is said to have entered the United States via a shipment of logs from France in the 1920s and causes what became known as Dutch elm disease, because it was first discovered in the Netherlands. Americans didn't help matters by planting rows and rows of elms close to one another so that the European elm bark beetles (Scolytus multistriatus) which are capable of carrying the fungus from one tree to another could help to easily infect many trees.
This scenario isn't only a problem of the past. "Today, we have an even bigger problem with pathogens getting transported from one country to another," says Britton. "We have a lot more international trade and exotic diseases from all over the world that can get introduced here."