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HOW TO IDENTIFY AND MANAGE OAK WILT IN TEXAS


INTRODUCTION

Oak wilt, one of the most destructive tree diseases in the United States, is killing oak trees in central Texas at epidemic proportions. Oak wilt is an infectious disease caused by the fungus Catalysts Fagacearum which invades and disables the water-conducting system in susceptible trees. All oaks  (Quarks sop.) are susceptible to oak wilt to some degree, but some species are affected more than others. Red Oaks, particularly Spanish Oak (Q. texana), 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 Chinquapin 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. 

SPREAD OF OAK WILT

Spread Through Roots

     Live Oaks tend to grow in large dense groups with interconnecting roots. The fungus may be transmitted from one tree to another through these root connections. Root transmission is the only proven means of spread among Live Oaks. As a result, patches of dead and dying trees ( infection centers ) are formed. Infection centers among Live Oaks in Texas expand at an average rate of 50 ft per year, varying from no spread to 150 ft in any one direction. Occasionally, the oak wilt fungus is transmitted through connected roots between Red Oaks, but movement through roots is slower in Red Oaks and occurs over shorter distances than Live Oaks.



Establishment of New Infections   

      Red Oaks appear to play a key roll in the establishment of new infection centers. The oak wilt fungus may be spread overland by insect vectors and by man through movement of wood from infected Red Oaks to other locations. Specialized spore-producing structures called fungal mats from beneath the bark of certain Red Oaks in late fall and especially in spring but do not form on Live Oaks. Individual fungal mats attract many kinds of insects, the most important of which are believed to be the sap-feeding nitidulid beetles. The fungus may be transmitted by the small beetles as they emerge from mats to feed on fresh wounds on healthy oaks. Fungal mats are most commonly formed on standing trees, but the can also developed on logs, stumps, and fresh firewood cut from diseased Red Oak trees. 

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, isolation of the fungus in the laboratory may be required 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.
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Foliar Symptoms
      Leaves on diseased Live Oaks often develop chlorotic ( yellow ) veins that eventually turn necrotic ( brown ), a symptom called veinal necrosis. Defoliation may be rapid, and dead leaves with brown veins often can be found under the tree months after defoliation. Leaves may exhibit other patterns of chlorosis and necrosis, such as intrveinal chlorosis, marginal scorch, or tip burn, but these symptoms are less reliable that 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, at starting at the leaf margins and progressing inward.
Fungal Mats

Fungal mats are reliable indicators for diagnosis of oak wilt. They most often form in 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 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 diagnosis may by confirmed by isolating the fungus from the diseased tissues in the laboratory. Samples can be submitted to: Texas Agricultural Extension Service, Plant Diagnostic Clinic, L.F. Peterson Building, Texas A & M University, College Station, TX 77843-2132. A County Extension Agent, Texas Forest Service Forester or trained arborist should be consulted for proper collection and submission of samples.

DISEASE MANAGEMENT

      There are currently three primary approaches used for oak wilt management in Texas, Successful control usually depends on an integrated program incorporating measures from all three approaches. The first approach attempts to prevent the formation of new oak wilt centers by eliminating diseased Red Oaks, handling firewood properly, and painting wounds on healthy oaks. The second approach involves trenching or other measures to disrupt root connections responsible for root transmission of the pathogen. Finally, injections of fungicide propiconazole ( Alamo TM ) into individual, high-value trees to help reduce crown loss and extend the life of the tree. These measures will not cure oak wilt but will significantly reduce tree losses. 

Preventing New Infections     
 
      Infected Red Oaks that die in late summer or early fall should be cut and burned or buried by early fall or soon after discovery to prevent insects from transmitting spores from fungal mats that may form on these trees in the fall or following spring. If this is not possible, the trees should be injected with a herbicide or deeply girdled with an axe and stripped of all bark  2 to 3 ft above the soil. Drying of the wood before fall discourages formation of fungal mats.
       All wounding of oaks ( including pruning ) should be avoided from mid February to mid June. The least hazardous periods for pruning are during the coldest days in midwinter and extended hot periods in mid- to late summer. Regardless of season, all pruning cuts or other wounds to oak trees, including freshly cut stumps and damage surface roots, should be treated immediately with would paint to prevent exposure to contaminated insect vectors.   
        Transporting unseasoned firewood from diseased Red Oaks is a potential means of spreading the oak wilt fungus. Oaks wilt can not be transmitted by burning infected firewood, but fungal mats may form on firewood in storage. Presently no vectors have been proven to transmit the fungus from Live Oaks or other oak trees, but diseased wood from any oak species should never be stored near health oak trees unless precautions are taken. It is best to purchase wood that has been thoroughly dried for at least one full year. If firewood from diseased trees is stored near healthy oak trees, it should be covered with clear plastic with edges buried to prevent insects from leaving the pile.

Stop Spreading Through Roots

       Measures can be taken to break root connections between Live Oaks or dense groups of Red Oaks to reduce or stop root transmission of the oak wilt fungus. The most common technique is to sever roots by trenching at least 4 ft deep with trenching machines, rock saws, or ripper bars. Trenches more that 4 ft deep may be needed to assure control in deeper soils. Correct placement of the trench is critical for successful protection of uninfected trees. There is a delay between colonization of the root system by the fungus and appearance of symptoms in the crown. Therefore, all trees with symptoms should be carefully identified first.

Then, the trench should be placed a minimum of 100 ft beyond these symptomatic trees, even though there may be "healthy" trees at high risk of infection inside the trench. Trees within the 100 ft barrier, including those without symptoms, may be uprooted or cut down and removed to improve the barrier to root transmission.Tree removal should be initiated after trenching, starting with healthy trees adjacent to the trench and gradually working inward to include symptomatic trees. Oak wilt infections centers are more easily suppressed when treated early, before they become too large. Untreated trees immediate outside the treated area should be closely monitored for several years. If the pathogen appears to have crossed the barrier, the same measures   ( new trenches and treatment of trees within the barrier ) should be repeated while the disease site is still small.

                 

Fungicide Treatments    

     The Fungicide propiconazole ("KESTRELTM,  Quali-Pro or Alamo TM , propiconazole systenic fungicide or other labelled products for oak wilt injection) can be used as a preventative treatment to reduce oak wilt symptoms in Live Oaks when applied before infection.  Limited success may also be achieved in trees treated with therapeutic injections during the earliest stages of infection. The fungicide is injected into the tree's water conducting vascular system through small holes drilled into the root flairs at the base of the tree. Treatment success depends in the health condition of the candidate tree, application rate, and injection technique. Injections should be done only by trained applicators.  Fungicide injections does not stop root transmission of the fungus. This treatment, therefore, is used best in conjunction with trenching or to protect individual high-value trees in situations where trenching is impractical. Healthy Live Oaks at high risk of infection in advance of an expanding infection center are preferred candidates for injection. Foliar symptoms can be used in selecting trees as candidates for preventative or therapeutic treatments. A tree with foliar symptoms of oak wilt, as well as non- symptomatic trees immediately adjacent to a tree with symptoms, should receive a therapeutic treatment. If symptoms are observed in more than 30 percent of the crown, it is unlikely a fungicide injection will be effective. Injections of nonsymptomatic trees at greater distances from the symptomatic tree (e.i. 75 to 150 ft ) will yield the best results for preventative treatment. 

                     
         There are several steps in the injection process that require careful attention following tree selection. Mixing the fungicide solution, exposing and drilling holes in the flair roots, connecting the injection apparatus to the tree, and monitoring uptake must be done according to label specifications and directions. Treatment may take several hours. Information and training are available through county extension offices. The services of a professional arborist may be required to assure proper injection.

Propiconazole - How does it work? 

Sterol biosynthesis:   Sterols are essential compounds in the cells of all creatures, components of cell membranes and other important anatomical features.  Propiconazole possesses systemic and some curative properties against certain diseases and prevents fungal cell formation and growth. Propoiconazole is absorbed into the fungus where its two modes of action attack fungal cells at several sites altering the cell, thereby inhibiting sulfur-containing enzymes and disrupting fungal energy production and prevents spore germination. Post-infection activity may limit disease establishment during early stages of development.


 

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INTEGRATED OAK WILT MANAGEMENT

        Early detection and prompt action are essential for successful management of oak wilt. The specific measures taken depend on several circumstances outlined in this brochure but should include appropriate combination of the following:
 

1. Prevent New Infections
         * Cut and dispose of disease Red Oaks immediately.
         * Avoid wounding oak trees, including pruning, from mid February through mid June, and   paint all wounds and fresh stumps regardless of season.
         * Handle oak firewood cautiously, burn all firewood before spring, and never store unseasoned oak wood from infected trees near healthy oak trees.
         * Cover unseasoned firewood ( from infection centers and unknown origins ) with clear plastic, and bury the edges of the plastic
2. Stop Spread Through Roots
          * Install a trench at least 4 ft deep and 100 ft beyond the perimeter of infection centers to break up root connections.
          * Cut or uproot all trees within the 100-ft barrier ( except those injected with fungicide )

 

3. Inject High-Value Oaks with Fungicide
          * Identify susceptible, high-value oak trees in proximity to expanding oak wilt infection centers.
          * Consult a trained and licensed arborist ( with certified applicators' license ) for treatment of susceptible tree with injections of propiconazole ( Alamo TM ). 

AUTHORS:
David N. Appel, Department of Plant Pathology & Microbiology, Texas A&M University, College Station, TX 77843-2132 
R.Scott Cameron, Texas Forest Service Lufkin, TX 75902-0310
A.Dan Wilson, Southern Hardwoods Station, Stoneville,MS 38776-0227
Jerral D Johnson, Texas Agriculture Extension Service College Station, TX 778-2132
 
The source material for this article covers excerpts from the United States Department of Agriculture, Forest Service,  
Southern Research Station, New Orleans, LA.  Brochure,  How -To SR-1
This brochure has been translulated into Spanish.
Titled: COMO IDNETIFICAR Y MANEJAR EL MARCHITAMENTO DEL ENCINO

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