Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
3/13/00
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579,
mgleason@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning March 17
Scots Pine Declared Loser in Midwest Landscapes
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
The Iowa caucuses have come and gone, dashing the prospects of several wannabe Presidents. Another Iowa hopeful, the Scots (a.k.a. Scotch) pine, also is faring poorly in this election year. The Scots pine's once-secure niche in the Iowa landscape is eroding, courtesy of a beetle and a little bitty worm that join forces to cause a big problem called pine wilt.
Who cares about Scots pine? This Scottish import has many virtues. Like other Scots, it's tough and resilient. Since its introduction to the Midwest over a century ago, Scots pine has thrived in Iowa's punishing extremes of soil and climate, where some other conifers perished ignominiously. Its long life, pleasing shape and eye-catching orange bark made it a favorite of landscapers and homeowners. But time and pests are catching up with Scots pine.
In the late 1970s, a new disease appeared on Scots pine in Illinois and Missouri. Needles abruptly dried and turned brown, either on scattered branches or on entire trees. The victims typically died within a few weeks to a few months. Groves and windbreaks succumbed with alarming speed. The epidemic was traced to a microscopic, worm-like animal called a nematode.
It turned out that this nematode, called the pinewood nematode, proliferated at warp speed inside the water-conducting resin ducts of Scots pine during hot summer weather. Pretty soon, there were billions of nematodes blocking the passages. Water flow shut down, and the tree died.
But the pinewood nematode was only half of the story. The other villain was the pine sawyer beetle, a wood-boring insect with an irresistible appetite for Scots pine. This unlikely pair made a formidable team. The beetle, a strong flier, carried the nematodes as hitch-hikers from dead and dying trees to healthy ones. When the beetles began munching on succulent young twigs, the nematodes vacated their flying taxi and wriggled into the feeding wounds. Once the nematodes found the resin ducts and started having babies, the tree's fate was sealed.
Attempts to curb pine wilt with insecticides and nematicides had disappointing results. The disease kept spreading. Nearly all of its victims were Scots pines, but Austrian pines would occasionally succumb, too, and rarely a white pine. The most effective weapon against pine wilt turned out to be a low-tech one: sanitation. In other words, promptly cutting down a diseased tree and chipping, burning, burying, or removing the wood sometimes stopped the spread of the disease to nearby trees. Sanitation worked because it denied the sawyer beetle a place to go through its life cycle and pick up pinewood-nematode riders.
In the more than 20 years since the first outbreaks, we've learned more about the nematode and the pine sawyer beetle. One lesson was that pine wilt seldom attacks pines less than 8 to 10 years old, so it posed little threat to Scots pine Christmas tree plantations.
Unfortunately, no new tricks have emerged to combat pine wilt in the landscape. Instead, the disease spread to Scots pine stands throughout Iowa, western Kentucky and eastern Kansas and Nebraska. By the late 1990s, the problem had become so severe in Iowa that ISU Extension specialists began to recommend against planting Scots pine as a landscape tree in Iowa - unless the owner was content with a tree that was unlikely to survive more than 15 to 20 years.
Why has pine wilt singled out Scots pine? Nobody has a clear answer, but there are some educated guesses. Scots pine is native to the moist, cool environment of Scotland. The Midwest's heat and drought are nothing like home for this import. The pinewood nematode is thought to be native to the Midwest, but it poses little threat to native conifers like white pine. In Japan, on the other hand, the pinewood nematode arrived by accident on logs from the United States and promptly devastated immense stands of native Japanese red and black pine. Like many severe disease epidemics, pine wilt involves an immigrant - whether it's a tree, like Scots pine in the Midwest, or the nematode, as in Japan.
We're still waiting for a happy ending to this story. The best defense we have now is sanitation. To stop the disease from spreading, take out dying trees promptly. With some luck, and some distance from other infected trees (sawyer beetles can easily fly distances of several hundred yards), sanitation may keep your remaining Scots pines healthy for years. If you'd like to learn more about pine wilt, a new publication on the disease will be available at County Extension Offices in Iowa, Missouri, Kansas, and Nebraska within a month.
If you're considering planting evergreen trees in your landscape, and you hope they will stay healthy for many decades, Scots pine is no longer a good bet for Iowa and the other pine-wilted areas of the Midwest. Consider white pine, or one of several species of spruces, firs and cedars that perform well in many Midwest sites.
ml: isugarden
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.