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A silent killer is poised to clear the Midwest landscape of one of its majestic pines.

Pine wilt disease, first identified in Iowa over 20 years ago, is killing the scot pines, also known as scotch pines. These trees had been heavily planted in rural shelterbelts since the 1930s.

The disease, which can kill a tree within weeks of exposure by the attacking organism, leaves the telltale signs of tan or brown colored needles.

Pine wilt disease is caused by a group of organisms acting in conjunction with one another - nematodes, sawyer beetles and bark beetles.

The pinewood nematode is a microscopic worm-like animal which feeds on both living plant cells surrounding the water-conducting passages of scot pines and the blue-stain fungi that live in the wood of dead or dying pines.

As the nematodes destroy the water-carrying canals in scot pines, the canals become clogged and resin flow stops. At that time , the symptoms of pine wilt appear and the tree begins to die. Depending on the damage done by the nematodes, the tree could die quickly in just a few weeks or over several months' time.

But the nematode couldn't spread pine wilt disease without assistance. The female pine sawyer beetle lays its eggs under the bark of dead or dying trees. The grubs form pupae, mature and then adult beetles emerge from the tree usually between late spring and early fall.

While the sawyer beetle is maturing, so is the nematode larvae which moves into the trachea or breathing mechanism of the beetle.

When the beetle emerges from the tree and flies away, it carries the hitchhiking nematode along on its travels. The sawyer beetle is a strong flier and that contributes to the spread of pine wilt in the Midwestern states. Hit the hardest are Iowa, Illinois, Missouri, Kentucky, eastern Kansas and southeastern Nebraska. Infestation has also been reported in Indiana, Ohio and Minnesota.

Once it finds another host, the young sawyer beetle begins to feed on young twigs. This feeding does little to damage the tree but it does create

entry wounds in which the nematode enters the tree and the cycle of destruction continues. If a sawyer beetle lands on a resistant pine, the nematode dies and the cycle ends.

The third villain of this trilogy of destruction is the bark beetle which is attracted to dying trees. As the bark beetle burrows through the dying bark, its body carries blue-stain fungi along with it. The fungi,

which is the staple of a nematode diet, rapidly colonizes the wood of a

dying tree. The presence of the fungi is evident by the cobalt-blue color it leaves behind on the wood.

Identified in Iowa 20 years ago by State Forester Wayne Fuhlbrugge in the Shimek Forest in southeast Iowa, pine wilt has continued to move through the state.

Fuhlbrugge is now serving as the district forester for a 17-county area in central Iowa for the Iowa Department of Natural Resources' Bureau of Forestry and he continues to monitor the spread of pine wilt.

"The scot pine is native to Europe - England and eastern Europe," said Fuhlbrugge, but pine wilt is the consequence of taking a plant out of its native habitat.

"Pine wilt is not a problem in Europe," said Fuhlbrugge. But like the chestnut or elm trees taken from Europe and planted in the U.S., scot pines are experiencing a blight once it was removed from its natural surroundings where it had a resistance to disease.

Scot pines were first used in the Midwest to help establish shelterbelts in an effort to fight off the land erosion of the Dust Bowl in the 1930s. The tree climatized well to the Midwest and it grew rapidly, said Fuhlbrugge.

But today it is those towering trees, some up to 80 years in age, that are more susceptible to pine wilt compared to the younger trees. Fuhlbrugge said that many trees less than 20 years of age seem to be immune to the disease.

Even with the threat of pine wilt, the scot pine continues to have several landscape applications, says Fuhlbrugge and the versatility of the scot pine lends it to be used on a restricted basis. For instance, young

saplings are used as trainer trees to help surrounding trees grow straight and vertical, he explains.

The young scot pine's age-resistance to disease appears to save Christmas tree farms from scot pine loss, according to an extension service collaborative study between the University of Missouri, University of Nebraska, Kansas State University, and Iowa State University.

Why a young tree can escape damage while an older tree falls or why one tree in a devastated grove will survive is unknown.

"It is the nature of nature," said Fuhlbrugge of the randomness of nature. As an illustration, he cites the phenomena that many of Dutch elm trees survived for decades while disease ravaged that tree in the 1970s.

Pine wilt is prevalent in the Midwest because of climate conditions,

but scot pine loss is not limited to the United States.

The Japanese black and Japanese red forests of that country have been

devastated by pine wilt in the past 100 years. Within the last 20 years, pine wilt has been identified in China. And in Korea and Taiwan pine wilt, has been identified in forests within the last decade.

The most effective way to stop pine wilt is by destroying dead or dying pines, but Fuhlbrugge cautions that a diagnosis should be done before taking action.

"Many times needle loss is normal," he said and proper tree identification should be done to insure the targeted tree is a scot pine and not another species.

Tree owners are encouraged to bring a limb sample to the district forester's office in Webster City, located at 723 Seneca Street for

evaluation.

When a pine wilt diagnosis is confirmed, tree owners are encouraged to destroy the tree quickly to help prevent further spread. Burning a tree is one way to disrupt the cycle, although using the wood for firewood is discouraged as sawyer beetles will continue to emerge from logs.

Another way to destroy a diseased tree is by chipping the wood. The wood chips need to be aged six weeks before use and contact between chips and fresh wounds on a tree should be avoided.

In addition, chips should not be piled against tree trunks or mixed into the soil of newly planted trees, Fuhlbrugge said.

Contact Teresa Wood at editor@freemanjournal.net or call 832-4350.