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Why are my trees dying?

May 1, 2013
Ips beetle

Ips engraver beetles are common pests of pines in Arkansas. (Image courtesy William M. Ciesla, Forest Health Management International, Bugwood.org)

Dr. Jon Barry, extension forester for the University of Arkansas System Division of Agriculture, on trees and drought.

Drought has been a hot topic for the last several years in Arkansas. County agents and extension foresters have received numerous phone calls about lawn trees dying due to insect and disease problems. Almost all of these problems trace back to one simple factor – drought.  Drought places trees under stress which may kill trees or may make trees more susceptible to insects and diseases.

A return to normal rainfall might lead landowners to conclude that trees should recover from drought and that they should expect to no longer see trees dying.  Unfortunately, this is not the case.  Once a tree has been severely stressed by drought it may not fully recover for several years.  If it is infested by insects or infected with a disease, it often will die sooner or later.  It’s that “later” part that gives many people a false hope that their trees will survive.

Severe drought greatly reduces photosynthesis in trees because they must conserve water to survive.  Without photosynthesis, trees don’t build adequate energy reserves to fully leaf out the following spring.  Our native trees are tough and can handle drought.  However, prolonged drought will deplete a tree’s energy reserves so severely that the tree may take several years to rebuild those reserves and fully leaf out again.

Hypoxylon canker in oaks has been one of the most common problems encountered in Arkansas’ trees through the last few years.  The fungus that causes hypoxylon is native to Arkansas and is always present at low levels in our forests.  If you have seen oaks with the bark flaking off to reveal a blue-gray surface, you have seen hypoxylon canker.  Even though hypoxylon is common, it is a weak pathogen that normally does not invade healthy trees.  However, when a tree is placed under stress by drought, the hypoxylon fungus can colonize the tree.  Once this happens, the tree will die within a few years.  That delayed death gives landowners the false hope.  There is no cure for the disease.  Even after a drought ends, trees infected by the hypoxylon fungus during the drought will continue to die for several years.

Hypoxolon canker on oak.

Hypoxylon canker on an oak. (Image courtesy Robert L. Anderson, USDA Forest Service, Bugwood.org)

Pines can be attacked by several species of native bark beetles.  Like the hypoxylon fungus, these beetles are always present in low numbers but usually kill only a few scattered pines each year.  Normally the pines killed by bark beetles are those trees under stress because of damage from storms or human activity.  However, when a drought stresses otherwise healthy trees; bark beetles, including the dreaded southern pine beetle, can attack many more trees.  Even after a drought ends, trees that were damaged by the drought will continue to be susceptible to bark beetles for a few years, thus landowners should not expect tree deaths to end immediately.

Most insect or disease problems in trees follow the same pattern.  The insect or disease is always present in the background but doesn’t become a problem until something puts the tree under stress.  Once the tree is weakened by stress, the pest attacks the tree almost always leading to tree death.

There are steps landowners can take to minimize tree deaths due to drought stress.  Homeowners should pay attention to what they do around yard trees.  Remember that 80% of a tree’s roots are in the top 18 inches of soil.  Be careful about digging around trees or compacting the soil.  Avoid covering soil with fill material or an impervious surface such as a sidewalk or paved driveway.  Forestland owners should maintain the vigor of forests by keeping them properly thinned.  Give yard trees some time to recover before deciding to replace them.  Some forestland owners are tempted to delay thinning their forest until timber prices rise.  In these cases the losses due to tree pests may exceed the revenue foregone by selling timber at current prices.

If you are having pest problems in your trees or if you would like more information about taking care of your woods, contact your county extension agent.

 

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