Posted 16 March 2008. PMN Crop News.
Researchers Continue to Look for Answers to Chestnut Blight
Michigan State University. www.canr.msu.edu
East Lansing, Michigan (March 2, 2009)--The destruction caused by chestnut blight in the eastern United States was nothing short of an ecological disaster. A few trees out of the millions that once played such a dominant role in eastern forests still struggle to survive, but seedlings and new sprouts from old stumps continue to become infected by the pathogenic fungus Cryphonectria parasitica.
Michigan is outside the natural range of the American chestnut, but scattered groves and single trees were growing here as the blight was working its way through eastern forests, and eventually it infected trees here, also. They didn’t all die, however, and when researchers’ attention was finally drawn to them, the obvious question was why not?
Researchers discovered not only the extremely virulent strain that had killed so many trees but also a less virulent strain that infected trees but didn’t kill them. Hopes that it would be the silver bullet that would give the American chestnut tree an edge against the fungus were not entirely dashed, but each new discovery about the cause of the reduced virulence (hypovirulence) and how it spread raised more questions.
“Early on, some researchers thought that maybe trees had developed resistance to the fungus,” says Dennis Fulbright, chestnut researcher at Michigan State University, who along with his colleague Andrew Jarosz, associate professor of plant biology, has been studying the effects of chestnut blight on chestnut trees in Michigan. “Others suggested that maybe it was the fungus that had changed. Experiments showed that when hypovirulent and virulent strains touched one another, the virulent strain would become less aggressive. While some scientists set out to use hypovirulent strains as treatment for infected trees, others were trying to find out just what was being transferred between strains. Eventually, three unusual viruses were identified.
“Unfortunately, these viruses don’t circulate the way the common cold virus does,” Fulbright observes. “It takes fusion of the virulent and hypovirulent fungi to transfer the virus.”
Genes in the fungus that control which fungal strains may exchange cellular materials such as hypoviruses may limit the transfer, he points out.
In other words, contact between virulent and hypovirulent strains may convert the virulent strains to less aggressive ones. Or it may not.
Treatment efforts predictably had mixed results. In one stand in Wisconsin, chestnut blight could infect trees faster than researchers could treat them. After a few years, however, it did appear that treated trees were faring better than untreated ones – they developed fewer lethal cankers than untreated trees. Researchers did not find the degree of dissemination between trees that they hoped for, however, and trees continued to suffer lethal infections. Some trees continue to survive with nonlethal cankers under their bark, and sprouts from dead and dying trees have at least some chance of being infected by hypovirulent strains that may protect them against the more aggressive strains.
“The fact that naturally occurring hypovirulent strains in the United States and Europe are offering some protection against the blight is encouraging,” Fulbright says. “The results of releasing hypovirulent strains to protect trees have been mixed. But there is more reason for hope now than there was in the early 20th century, when the certain predictable outcome of the blight was death. Today the outcome is at least less predictable, and time will tell whether hypovirulence will be more than just a footnote in the story of chestnut blight in America.”