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Sick as a dogwood: Insects, disease mute fall colors

By Debra Alban
CNN

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(CNN) -- You're driving up the East Coast and admiring the fall colors. But something seems off.

You ask yourself, "Have there always been this many dull brown leaves in the mix?"

Experts say the right balance between warm and cool temperatures and a good amount of rainfall makes 2006 a great year for autumn color in the Northeast.

But that's not to say it's going to be perfect. Disease and insect infestation can keep a tree from looking its best or even kill it.

The fall webworm was hard at work in the southern Appalachians, where Fred Hain, professor of entomology and forestry at North Carolina State University does most of his research. Infestation by the insect damaged as much as half of the hardwood trees this season, he said, based on a casual observance while driving through that area.

And the New York Botanical Garden is suffering a devastating blow.

"We're currently losing an American elm to Dutch elm disease, and that's a huge bummer for us at the Garden," said Jessica Arcate, associate curator of trees and shrubs.

On the bright side, when it comes to tree diseases that affect fall color, you don't really have to worry about huge clusters of trees being affected.

"Any of these diseases occur in a spotty sort of fashion," said Dr. Kevin Smith, project leader and plant physiologist for the Northern Research Station, part of the United States Department of Agriculture (USDA) Forest Service. "So it isn't like you can say, 'This particular vista or view is 10 percent less than it should be.' "

It's also difficult to accurately quantify the number of trees with compromised foliage from one year to the next. The amount of rain, temperature variations and insect behavior are key factors in tree health, experts say.

Blights and diseases

Joe Laforest, a forest health specialist at the University of Georgia, laughed as he recalled a visitor to the Dow Gardens in Midland, Michigan, who asked him, "What kind of tree has those polka-dotted leaves?" They were her favorite, she told him.

In fact, the woman had noticed a number of fallen maple leaves with tar spot, a common fungal disease that doesn't often elicit such a positive reaction.

Tar spot on maple is "basically just a big black blotch that forms on the leaf," Arcate said. Tar spot on maple also leads to the trees dropping their leaves earlier than normal, she said.

Tar spot is one type of blight, but "there are a whole host of fungi that make a living [supporting] their nutrition by infecting leaves, and in essence competing with the tree for that sugar," Smith said.

Maple anthracnose is another common blight that can cause premature defoliation and browning of leaves, Smith said, as well as making them appear "perhaps even a little scorched - as if you took a little propane torch and heated the leaves around the edges."

So how do trees come down with these illnesses?

"Most of the diseases are propagated by wet weather splashing the spores around onto the leaves," said Patrick Parker, plant health care program director for SavATree, a tree, shrub and lawn care provider in the Northeast.

With the amount of rain there's been this year in the region, "we've seen a lot of anthracnose in a lot of different species: oaks, maples, dogwoods, sycamores, just about everything this year has had some small amount of anthracnose," Parker said.

More detrimental to trees than aesthetically displeasing tar spot and anthracnose are vascular diseases, which can kill a tree.

With American chestnut blight, for example, "the fungus just clogs up the vascular system completely... From a person perspective, it's... as if you couldn't have your blood flowing," Arcate said.

In fact, about a century after the blight was known to have begun infecting the American chestnut, large chestnut trees have been nearly wiped out from the American landscape, Smith said.

Insects

"Any time a tree has an infestation of bugs, it limits its ability to grow leaves," says Roger Cook, landscape contractor for PBS's "This Old House."

With the heavy loss that insects can cause for fall color - and trees in general - it may be a comfort to tree-lovers that these things come in cycles.

"One year is a bad year, parasites and predators and diseases and natural enemies catch up to it, and so the following year probably won't be as bad," Hain said.

Three classes of insects contribute to the less appealing leaf colors, said Therese Poland, a research entomologist for the Forest Service.

Gypsy moths, part of the defoliator class, chow down on leaves in the spring when the foliage is just starting to grow, Hain said.

Trees defoliated by gypsy moths "may put out a new set of leaves, but if they do, those leaves aren't going to be anything comparable to the original set of leaves that were eaten off the tree," Cook said.

And then there are those fall webworms.

Large groups of these caterpillars feed on leaves and spin webs on them, Poland explained.

"At this time of year when there's a lot of webbing, it's quite unsightly," she said.

Wood-borers, the second class which is mostly comprised of beetles, cause crown thinning and defoliation, Poland said. They complete part of their life cycle in the tree bark or in bark crevices. In doing so, "they basically disrupt the ability of the tree to trans-locate water and nutrients. And so then leaves begin to turn yellow... and start falling from the branches," Poland said. "Heavily attacked trees can die within one to three years."

Stem and petiole borers, the third class, will tunnel - or bore - into small twigs or the leaf petiole, which is the stalk that attaches the leaf to the stem, and then cause the leaf to fall off the tree, but not before the leaves turn "kind of yellowish in color" over the summer, Poland said. But beyond poor fall coloration, the presence of this type of borer can kill trees, she adds.

Poland noted that if a tree is in rough shape to begin with, that could mean it's got more than just one strike against it. "Many of these insects respond to tree stress."

"In the case of emerald ash bore, for instance, we have found that [trees under stress already] are more attractive... The beetles can somehow cue in and say, 'This tree is less resistant,' " Poland said

What you can and can't do

If you have trees on your property that you'd like to protect from illness, you can take preventative measures, such as watering or fertilizing them, Parker said. But "generally once you see the condition, there's nothing you can do for it that season," he adds.

On the other hand, if a large population of trees in a tourist area where income depends on spectacular fall color is infested with gypsy moths, "if you sprayed it would cost you more dollars than the tourism would probably bring in... Unfortunately, these tourist areas are resting on Mother Nature to do the work for them," Cook said.

"There's nothing greater than for a family to go out and relax, take a drive, picnic and enjoy the last display of fall Mother Nature puts on," Cook said. "Even if it's at 40 percent or 50 percent, that's pretty darn good."

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