How to Identify Oak Wilt in Texas Full StoryIntroductionOak wilt, one of the most destructive tree diseases in the United States, is killing oaktrees in central Texas at epidemic proportions (fig. 1). Oak wilt is an infectious disease caused bythe fungus Ceratocystis fagacearum, which invades and disables the water-conducting system insusceptible trees. All oaks (Quercus spp.) are susceptible to oak wilt to some degree, but somespecies are affected more than others. Red oaks, particularly Spanish oak (Q. buckleyi), Shumardoak (Q. shumardii), and blackjack oak (Q. marilandica), are extremely susceptible and may playa unique role in the establishment of new oak wilt infections. White oaks, including post oak (Q.stellata), bur oak (Q. macrocarpa), and chinkapin oak (Q. muehlenbergii), are resistant to thefungus and rarely die from oak wilt. Live oaks (Q. virginiana and Q. fusiformis) are intermediatein susceptibility to oak wilt, but are most seriously affected due to their tendency to grow fromroot sprouts and form vast interconnected root systems that allow movement (or spread) of thefungus between adjacent trees. The successful management of oak wilt depends on correctdiagnosis and an understanding of how the pathogen spreads between different oak species.1 U.S. Department of Agriculture, Forest Service, Southern Research Station, New Orleans, LA, Revised June 2003,How-To SR-1.Identification of Oak WiltFoliar symptoms, patterns of tree mortality, and the presence of fungal mats can be usedas indicators of oak wilt. However, laboratory isolation of the fungus is recommended to confirmthe diagnosis. A trained expert should be consulted when in doubt.Patterns of Tree MortalityMost live oaks defoliate and die over a 1- to 6-month period following initial appearanceof symptoms. Some live oaks take longer to die, and a few untreated trees may survive manyyears in various stages of decline. A few live oaks in oak wilt centers escape infection or may beresistant to the fungus and apparently remain unaffected by the disease.Red oaks seldom survive oak wilt and often die within 3 to 4 weeks following the initialappearance of symptoms. During summer months, diseased red oaks often can be spotted from adistance because of their bright autumn-like coloration in contrast to the surrounding greenery.Foliar SymptomsLeaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turnnecrotic (brown), a symptom called veinal necrosis (fig. 2). Defoliation may be rapid, and deadleaves with brown veins often can be found under the tree for months after defoliation. Leavesmay exhibit other patterns of chlorosis and necrosis, such as interveinal chlorosis, marginalscorch, or tip burn, but these symptoms are less reliable than veinal necrosis for diagnosing oakwilt in live oaks.Foliar symptoms of oak wilt on red oaks are less distinct. In early spring, young leavessimply wilt, turning pale green and brown. Mature leaves develop dark green water soakingsymptoms or turn pale green or bronze, starting at the leaf margins and progressing inward.Fungal MatsFungal mats (fig. 3a, b) are reliable indicators for diagnosis of oak wilt. These specializedspore-producing structures most often form in the spring on red oaks that developed advancedsymptoms of oak wilt the previous late summer or fall. Red oak infections in late spring andsummer usually do not give rise to fungal mats due to high temperatures and low soil moistureconditions. Fungal mats can be found by looking for inconspicuous narrow cracks in the bark ofdying red oaks leading to hollow areas between the bark and wood. They often have a distinctiveodor similar to fermenting fruit. Fungal mats can be exposed for inspection by chopping awaythe loose bark.Laboratory DiagnosisOak wilt diagnoses may be confirmed by isolating the fungus from diseased tissues in thelaboratory. Samples can be submitted to: Texas Plant Disease Diagnostic Laboratory, 1500Research Parkway, Suite A130, Texas A&M University Research Park, College Station, TXTexas Oak Wilt ORGCALL Us TODAY for a FREE Inspection.
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How to Identify
Oak Wilt in Texas
Full Story
Introduction
Oak wilt, one of the most destructive tree diseases in the United States, is killing oak
trees in central Texas at epidemic proportions (fig. 1). Oak wilt is an infectious disease caused by
the fungus Ceratocystis fagacearum, which invades and disables the water-conducting system in
susceptible trees. All oaks (Quercus spp.) are susceptible to oak wilt to some degree, but some
species are affected more than others. Red oaks, particularly Spanish oak (Q. buckleyi), Shumard
oak (Q. shumardii), and blackjack oak (Q. marilandica), are extremely susceptible and may play
a unique role in the establishment of new oak wilt infections. White oaks, including post oak (Q.
stellata), bur oak (Q. macrocarpa), and chinkapin oak (Q. muehlenbergii), are resistant to the
fungus and rarely die from oak wilt. Live oaks (Q. virginiana and Q. fusiformis) are intermediate
in susceptibility to oak wilt, but are most seriously affected due to their tendency to grow from
root sprouts and form vast interconnected root systems that allow movement (or spread) of the
fungus between adjacent trees. The successful management of oak wilt depends on correct
diagnosis and an understanding of how the pathogen spreads between different oak species.
1 U.S. Department of Agriculture, Forest Service, Southern Research Station, New Orleans,
LA, Revised June 2003,
How-To SR-1.
Identification of 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 trained expert should be consulted when in doubt.
Patterns of Tree Mortality
Most live oaks defoliate and die over a 1- to 6-month period 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. A few live oaks in oak wilt centers escape infection or may be
resistant to the fungus and apparently remain unaffected by the disease.
Red oaks seldom survive oak wilt and often die within 3 to 4 weeks following the initial
appearance of symptoms. During summer months, diseased red oaks often can be spotted from a
distance because of their bright autumn-like coloration in contrast to the surrounding greenery.
Foliar Symptoms
Leaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turn
necrotic (brown), a symptom called veinal necrosis (fig. 2). Defoliation may be rapid, and dead
leaves with brown veins often can be found under the tree for months after defoliation. Leaves
may exhibit other patterns of chlorosis and necrosis, such as interveinal chlorosis, marginal
scorch, or tip burn, but these symptoms are less reliable than veinal necrosis for diagnosing oak
wilt in live oaks.
Foliar symptoms of oak wilt on red oaks are less distinct. In early spring, young leaves
simply wilt, turning pale green and brown. Mature leaves develop dark green water soaking
symptoms or turn pale green or bronze, starting at the leaf margins and progressing inward.
Fungal Mats
Fungal mats (fig. 3a, b) are reliable indicators for diagnosis of oak wilt. These specialized
spore-producing structures most often form in the spring on red oaks that developed advanced
symptoms of oak wilt the previous late summer or fall. Red oak infections in late spring and
summer usually do not give rise to fungal mats due to high temperatures and low soil moisture
conditions. Fungal mats can be found by looking for inconspicuous narrow cracks in the bark of
dying red oaks leading to hollow areas between the bark and wood. They often have a distinctive
odor similar to fermenting fruit. Fungal mats can be exposed for inspection by chopping away
the loose bark.
Laboratory Diagnosis
Oak wilt diagnoses may be confirmed by isolating the fungus from diseased tissues in the
laboratory. Samples can be submitted to: Texas Plant Disease Diagnostic Laboratory, 1500
Research Parkway, Suite A130, Texas A&M University Research Park, College Station, TX
Texas Oak Wilt
ORG
CALL Us TODAY for
a FREE Inspection.
