Drought leaves weakened trees susceptible to pests, diseases for years

COLLEGE STATION — The drought of 2011 appears to be fading as far as rainfall totals go, but less obvious impacts may continue for years as weakened trees fall victim to pests and disease, tree experts note.

“One thing that most people don’t realize is that this drought is going to have an impact on the trees that were damaged in 2011 for probably five to seven years from now,” said Dr. David Appel, Texas A&M AgriLife Extension Service plant pathologist. “And that’s assuming that we get normal rainfall. If we have another drought, that could  set them back even further.”

The red oak was impacted by the drought of 2011. Texas A&M AgriLife Extension Service specialists say the drought will have lingering effects from the drought for perhaps seven years, even if normal rainfall occurs. (Texas A&M AgriLife Extension Service photo by Robert Burns)

Appel, who specializes in trees, said certain diseases are going to be on the increase because of the drought.

The Texas A&M Forest Service estimates 301 million rural trees and 5.6 million urban trees died from the 2011 drought and its impact.

“The visual manifestation of the drought includes limb dieback, the crown is going to be asymmetric or lopsided, growth is going to be reduced and there will be smaller, more yellow leaves,” Appel said of stressed trees. “All of this is a sign that the trees were depleted of their carbohydrate reserves during the drought, and they just aren’t able to carry on the normal growth that they would in normal years.”

He said tree owners may also see infection by certain kinds of fungi, which can be dangerous.

“These infections may reduce the structural integrity of the tree,” Appel said. “You  can start to have these limbs and branches fall off, and they can hurt people.”

Knowing whether a tree is dead or dying can be hard to tell, he noted, and one may want to wait until next spring to examine leaves and overall health or call a tree expert before cutting.

However, he said there are a few simple things one can do to test tree health.

“Do a twig test. Take the twigs and bend them. If the twig is pliable, then you know it’s still alive and it’s just going through normal senescence,” Appel said. “However, if the twig breaks then you can be sure that that twig is dead and those are the kinds of branches that need to come out of the tree.”

    One disease having an impact currently is Hypoxylon canker, he said.

“You might see this on a water oak where the bark sloughs off and it has this light gray coating just underneath the outer bark,” Appel explained. “If that’s coming out, then the tree is dead, and it definitely should come down. Once this fungus starts growing into the wood, it greatly reduces the structural integrity and the tree can become dangerous.”

He said this is also showing up on sycamore – with large black marks on the trunk – and some other types of trees.

“Generally we recommend to get an expert up there not only because they know what to look for and take out, but always there is also the safety concern of having people climb around in their trees without the proper equipment,” he added.

Otherwise, Appel suggested, continue to watch trees until next spring, and the next spring and the next spring, because there isn’t a lot that can be done for trees in this condition.

    “Research has shown that the carbohydrate reserves in these trees are still depleted to five, six, seven years after the drought occurred, even if we get good weather and rain,” he said. “So it takes awhile for the trees to come back before they’re going to be able to resist all the insects and fungi that normally do them no harm and before they’re going to get back to normal growth.”

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