It seems weather, more than any other factor, ushers in pests and diseases that leave trees vulnerable to infection.
In Denver, where temperatures may swing by as much as 50 to 70 degrees in one day, tree diseases usually “aren’t a huge thing,” says Steve Geist, senior consulting arborist with Swingle Lawn, Tree & Landscape in Denver. But in 2012, Denver saw an unusually hot spring.
“In 2012, we ran into an uptick of Pine Wilt Disease,” Geist says. The disease is caused by the Pine Wood Nematode, a microscopic, wormlike animal. “Pine Wilt is the only tree disease caused by an animal,” he says.
“It’s very peculiar and difficult to work with,” Geist says. “It’s a major pest in the Midwest: Illinois, Missouri, Central and Eastern Nebraska, and Kansas has got quite a bit of it. It gets going a lot faster if you have a hot spring, which we did in 2012. We were running into quite a few deaths of Austrian and Scots pine, exotic pines that aren’t native."
To treat it, you inject the trees every other year with a product with the active ingredient abamectin, Geist says.
While the spring of 2012 was hot and dry, the spring of 2013 was cooler in Denver. “In 2013, we didn’t see a whole lot of it. It cooled off. It’s still here but it takes millions of these things inside a tree to choke it off. If it doesn’t get warm in the spring, the reproduction cycles are muted a little bit.”
Thousand Cankers disease, which attacks black walnut trees, has also been an issue in parts of Colorado.
“Thousand Cankers has wiped out walnut trees in several communities in the eastern plains of Colorado,” Geist says. “You cut the tree down. There is not a great solution for it. Soil drenches with insecticide are thought to slow the progression down.
"We use Merit, or post patent, anything with imidacloprid. It comes from a small tiny bark beetle and tens of thousands of these things attack a tree. They cause a fungus which causes a canker and the cankers coalesce and kill the tree.
“It is awful but we don’t have that many walnuts in Colorado. It keeps popping up in the Midwest and eastern states where the walnut is native. For us, it’s just sad. For them, they’ll have a huge problem.”
Rex Bastian, PhD, is a regional technical advisor for Davey Tree, and he is based in suburban Chicago. He also says that Thousand Cankers is a major threat to the walnut trees in the states where it’s native.
“They believe it came out of some of the walnuts that were in southwest U.S., Arizona, New Mexico … it showed up in Tennessee a year or so later then moved into Virginia, then Pennsylvania, and last year it spread into North Carolina and Ohio. These things, they get a little toehold and begin to spread out from there.”
In the Mid-Atlantic states, Bastian, whose territory stretches from Chicago into New York and Pennsylvania, says Boxwood Blight is also of concern.
“This disease is causing major issues and is likely to get a lot worse in future years, although most of these diseases can ebb and flow with weather conditions,” Bastian says.
Some universities, including North Carolina State, are working to identify fungicides to help with Boxwood Blight, Bastian says, but the treatments are preventative, not curative.
John Brewer is an arborist representative for Arborist Enterprises in Lancaster, Pa. In his region, just outside of Philadelphia, boxwood blight “is the newest disease” in the area. “We’ve been getting alerts from Penn State Extension. It’s only been in the area for about three years now, so it’s pretty new,” Brewer says.
“At this point, there’s no cure. It can kill a plant in about two weeks. Basically, you notify the extension agency and remove the infested material … We’re managing it more at this point. It’s an issue with nurseries. If they’re growing boxwoods for sale, then they have to wipe out the entire field, or fields, of boxwoods.”
Still, even when the disease is found, the removal process is one that must be done with caution so as not to spread the disease to other plants.
“If you’re trying to prune it out, you have a 10 percent bleach and water solution to spray on the pruning tools. You put infected cuttings in a landfill or burn them. And, after handling it, don’t go out and touch another boxwood,” Brewer says.
Weather has also contributed to the increase in conifer Needle Cast disease in the Midwest, Bastian says, “because of the rainy springs we’ve had.” The disease, he says, impacts spruce, pine and Douglas fir trees. The disease has become so prevalent, Bastian says, “that landscapers are not planting Colorado spruce anymore in our area” because of its susceptibility to it.
The disease attacks the needles of the plant, causes them to turn brown and drop off, “leaving a bare-looking tree, especially on the inside of the plant … now that the disease has taken effect, you can’t put the needles back. They don’t grow back in the same place,” Bastian says.
Treatment of the disease is difficult. “The disease is sneaky. Symptoms might not show up for 12 or 14 months and by then you may be a day late and a dollar short,” he says. Though treatments may be available, they can be costly, Bastian says, which makes the decision between treating the tree or removing it a cost/benefit analysis for a residential.
If there’s a trend in tree disease management, these experts say that rather than trying to combat some of these most insidious diseases, they’re seeing an increase in removing infested or vulnerable tree varieties and replacing them with newer, more resistant cultivars.
With Boxwood Blight, for example, Bastian says, there are many cultivars of Boxwoods that vary in their susceptibility to Boxwood Blight.
"Probably what we’ll see is that the ones that are more resistant, those species will be used more," he says.
"They won’t be planting the highly susceptible cultivars, but that takes time for nurseries to be able to gear up.”
The author is a freelance writer based in Mount Vernon, Washington.
Photos courtesy of Swingle, Dr. Ned Tisserat and Davey Tree