Oak Wilt

Oak wilt is a vascular disease of oak (Quercus sp.) caused by the fungus Bretziella fagacearum. The fungus is spread below ground via root graft transfer and above ground by nitidulid picnic beetles and possibly oak bark beetles. Members of the red oak family die rapidly (4-6 weeks) while members of the white and live oak family have more tolerance and may live longer. The disease is widespread throughout the eastern and central United States as well as Texas. Oak wilt is considered one of the most economically important diseases affecting oak trees in America.

Treatment Strategy

Red Oak Group

Treat only asymptomatic red oaks that are within root-grafting distance (50 ft) to an infected red oak. Alamo does not work on red oaks that are showing symptoms.

White Oak Group

Treat high value asymptomatic white oaks that are within root-grafting distance of infected white oaks. Symptomatic white oaks which display less than 30% canopy loss from oak wilt can also be treated successfully with Alamo. Treating trees with greater than 30% crown loss may reduce the success rate of an Alamo treatment. White oaks are more tolerant than red oaks and may survive for several years after becoming infected. Research has demonstrated that therapeutic treatments on white oaks are effective. Symptomatic trees can take longer to treat because xylem vessels may be occluded.

Live Oak Group

Treat asymptomatic live oaks that are within root-grafting distance (75 – 200 ft.) of infected live oaks. Live oaks are intermediate in resistance and typically die within 12 months of initial symptom expression, although a small percentage of the population of infected trees may live for extended periods of time in varying states of decline. Treat symptomatic live oaks which display less than 30% canopy loss from oak wilt. Treating trees with greater than 30% crown loss may reduce the efficacy of Alamo. Symptomatic trees may take longer to treat because xylem vessels may be occluded

Other Treatment Practices

  • Apply Propiconazole 1.3 ME T&O or T-Bird 4.5L for management of anthracnose.
  • Do not prune or wound trees
    • April through June in Northern States (delay until dormant season to further minimize risk)
    • February through June in TX
  • Plant resistant native species
  • Consider an injection of Mectinite (in conjunction with Alamo application) or soil application of Xytect to control related pests such as Two lined chestnut borer.

Tree Injection Using Alamo

Signs of Damage

Red Oak Group

  • Symptoms:
    • Individual leaves wilt from the tips and margins toward the base, turning bronze to brown
    • There is often a distinct line between living and dead leaf tissue
    • Wilting leaves may curl around the midrib and fallen leaves are often green at the base; sometimes leaves may fall while entirely green
    • Leaves may also have a water soaked appearance
    • Trees rapidly wilt from the top of the canopy downward, often completely within a few weeks
    • Sapwood streaking underneath the bark consistently appears in the red oak group
    • Oak wilt spreads mainly through the root systems of adjacent trees, so nearby dead trees are a good indicator that oak wilt is present
  • Signs:
    • Spore mats may be found on the trunk and large limbs of infected trees that are completely wilted or in advanced stages of crown wilt

White Oak Group

  • Symptoms:
    • Individual leaf symptoms are similar to those of red oaks
    • Rather than complete wilting from the top down, however, white oaks may lose a branch or two per year over an extended period of time until the tree dies
    • Sometimes leaf symptoms show up scattered throughout the canopy leading to decline over an extended period
    • Vascular discoloration beneath the bark is less consistent in white oaks; however, it is often easier to see discoloration in older annual rings
  • Signs:
    • The fungus typically does not produce spore mats in white oaks.

Live Oak Group

  • Symptoms:
    • Leaves on infected live oaks in Texas show a characteristic pattern quite different than in red or white oaks
    • Leaf veins may turn yellow or brown while the rest of the leaf remains green (veinal necrosis)
    • Other symptoms can include leaf tip burn and intraveinal chlorosis, although these can also be indicators of other problems like herbicide damage
    • Symptoms are typically found scattered throughout the tree and defoliation and death usually occur within a year; some trees may survive for many years in a severe state of decline
    • Sapwood streaking is not an indicator in live oaks
  • Signs:
    • The fungus typically does not produce spore mats in live oaks

Red oak leaf

Spore mat under red oak bark

White and live oak leaves

Trees At Risk

  • Red Oak Group
    • All species in the red oak group within the range of the disease are susceptible
    • Red oak(Q. rubra)
    • Swamp Spanish oak (Q. palustris)
    • Northern pin oak (Q. ellipsoidalis)
    • Black oak (Q. velutina)
    • Schumard oak (Q. schumardii)
    • Nuttall's oak (Q. texana)
    • Texas red oak (Q. buckleyi)
  • White Oak Group
    • All white oak species within the range of the disease are susceptible
    • White oak (Q. alba)
    • Post oak (Q. stellata)
    • Bur oak (Q. macrocarpa)
  • Live Oak Group
    • Texas live oak (Q. virginiana var. fusiformis (Syn. Q. fusiformis))

A Treatment Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments by forestryimages.org

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