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A nonprofit dedicated to trees is issuing a warning to Dallas citizens who own oak trees: Put down the pruning shears.

Homeowners and landscaping types may be tempted to do some spring pruning, but when it comes to oak trees, now is not the time.

The reason: oak wilt, a devastating, incurable fungus that has been slowly spreading over a large part of the U.S. This tree disease is contagious and can kill an oak in as little as two weeks.

This is why Texas Trees Foundation is urging homeowners and business owners to NOT prune oak trees after February 1, and to refrain from pruning them between February 1-June 30. (That’s the time window for North Texas. In cooler parts of the country up north, it runs from March to July.)

Do not prune unless there is an emergency, and avoid wounding your oak trees during this time.

Oak wilt is an infectious disease caused by the fungus Bretziella fagacearum, which invades and disables the water-conducting system (xylem) in oaks. Beetles who carry the disease are attracted to bark damage or “wounds” where tree limbs have been removed.

All oaks can be infected. However, some oak species are affected more than others.

Most susceptible oaks
Red oaks: Members of the red oak group, particularly Texas red oak (AKA Spanish oak), shumard oak, blackjack oak, and water oak are most susceptible to the fungus and may play a unique role in the establishment of new oak wilt infections.

Live oak and Texas live oak are intermediate in their susceptibility to oak wilt, but are most seriously affected due to their tendency to grow with vast, interconnected root systems that allow movement of the fungus among adjacent trees.

White oaks: Members of the white oak group include post oak, bur oak, Mexican white oak, white shin oak, durand oak, lacey oak, and chinquapin oak. Although white oaks show some tolerance of the disease, all oaks can be infected by the fungus. White shin oak, lacey oak, and chinquapin oak can grow in stands with interconnected root systems, enabling the fungus to possibly infect adjacent trees that are susceptible to the fungus.

How to detect oak wilt
Foliar symptoms, patterns of tree mortality, and the presence of fungal mats can be used as indicators of oak wilt. However, laboratory isolation of the fungus is recommended to confirm the diagnosis. A certified arborist should be contacted when in doubt.

Foliar (leaf) symptoms on live oaks include:

  • Yellow veins. Leaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turn necrotic (brown), a symptom called veinal necrosis. The most commonly seen foliar symptom on live oaks infected with oak wilt.
  • Vein banding. Vein banding is where the leaf vein is a darker green than the rest of the leaf.
  • Tip burn or margin burn, which turns the edges of the leaf brown.

Defoliation may be rapid, and dead leaves with brown veins often can be found under the tree for months after defoliation.

Patterns of tree mortality: Most live oaks defoliate and die within 3 to 6 months following initial appearance of symptoms. Some live oaks take longer to die, and a few untreated trees may survive many years in various stages of decline. Occasionally, a few live oaks in an oak wilt center may escape infection and remain unaffected by the disease.

Red oaks never survive oak wilt and often die within 4 to 6 weeks following the initial appearance of symptoms. During summer months, diseased red oaks can often be spotted from a distance because of their bright, autumn-like coloration in contrast to the surrounding greenery. This symptom is called flagging. So sad.

How to manage oak wilt
Awareness is important when identifying oak wilt. There are four primary approaches used for oak wilt management in Texas:

  • Prevent the formation of new oak wilt infection centers by eliminating diseased red oaks, handling firewood properly, proper timing of pruning, and painting wounds on healthy oaks.
  • Trenching or other measures to disrupt root connections responsible for root transmission of the pathogen.
  • Injection of the fungicide propiconazole into individual, high-value trees to help reduce crown loss and extend the life of the tree.
  • Plant other tree species to create diversity in the landscape and to mitigate the impact of oak wilt.

Successful control usually depends on an integrated program incorporating measures from all four approaches. These measures will not cure oak wilt but can significantly reduce tree losses.

How to slow the spread of oak wilt
Regardless of season, immediately paint all pruning cuts and other wounds to oaks. Any kind of wound dressing or paint can be used, and all are equally effective at preventing infections from fungal spores when applied immediately.

To repeat: Avoid pruning or wounding oaks from February 1-June 30.

Debris from diseased red oaks should be immediately chipped, burned, or buried.

When to prune
The least hazardous periods for pruning are during the coldest days in winter and extended hot periods in mid to late summer. The best time to prune oaks is during late fall.

Proper pruning techniques should always be used. These include making proper pruning cuts and avoiding injurious practices such as topping or excessive crown thinning. As a general guideline for pruning, clean all pruning tools with 10 percent bleach solution or Lysol between sites and/or trees.

The Texas Trees Foundation is a nonprofit tree planting organization dedicated to greening North Central Texas. Established in 1982, the Texas Trees Foundation manages the nation’s largest non-profit urban tree farm and plants trees on public property.

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