Identification and Control of Oak Wilt in Texas Urban Forests

Host Influence on Pathogen Transmission

Live oaks are superior landscape trees for urban environments in Texas. They grow on a wide variety of soils, tolerate wide temperature extremes, and thrive under varying levels of annual precipitation (13). The foliage on live oaks is dense and semi-evergreen, providing excellent shade and cover during all months of the year except March, when defoliation occurs. Live oaks reach maximum heights of 60 ft., and have large lower branches and spreading crowns. Overgrazing, fire control, and selective management have contributed to the formation of large expanses of nearly pure live oak stands throughout the woodlands of central Texas. Live oaks tolerate the site disruptions associated with urban expansion very well, so that existing native trees usually comprise the most popular shade tree in central Texas towns and cities. These native stands growing in lawns and boulevards are commonly supplemented with planted live oaks. In towns such as Austin, San Antonio, Waco, Ft. Worth, Dallas, and hundreds of smaller communities, the proportion of live oaks may far exceed 10%, the maximum level recommended to maintain a stable, healthy urban forest (17).

The abundance of live oaks in the native plant community relates in part to their habit of vegetative reproduction. Prolific root sprouting is a common trait of live oaks (26). Thousands of nuisance root sprouts are commonly observed in lawns and plantings. This characteristic is considered to be a selective advantage for trees colonizing disturbed sites under harsh environmental conditions (26). Many connections among trees are believed to be maintained through maturity. This development of a common root system, connecting numerous ramets as a clone, makes them highly vulnerable to a virulent, vascular inhabiting fungus such as C. fagacearum. Presumably, root grafts are common on the thin soils and further contribute to the risk of infection by a vascular parasite. Once the pathogen is introduced into a stand of live oaks by the insect vector, the focus of mortality expands rapidly from one tree to the next creating an ever growing patch of hundreds, and sometimes thousands, of dead and dying trees (8).

Sap-feeding nitidulid beetles (Coleoptera: nitidulidae) are suspected as primary vectors for overland transmission of C. fagacearum in Texas (6,7). The same group of insects have been also implicated as primary vectors in other states (16). However, the role of nitidulids in Texas is somewhat different. To obtain inoculum, nitidulids require formation of a fungal mat by the pathogen beneath the bark of diseased trees (16,24). Fungal mats attract the beetles with a strong, sweet odor and are covered with sticky spores which easily contaminate the insects when they feed and breed. Mats form on members of the red oak group (subgenus Erythrobalanus); they do not form on diseased white oaks (sub-genus Leucobalanus). In the Mid-Atlantic and North Central U.S., most diseased trees are potential mat formers because the primary hosts in those regions are red oaks. In contrast, mats have never been found on diseased live oaks, the predominant host in Texas. The opportunity for long distance spread is therefore limited in Texas by the proportion of red oaks growing in a disease center. The two most prevalent red oak species in Central Texas are Spanish oak (Q. texana) and blackjack oak (Q. marilandica). These species are extremely susceptible to C. fagacearum, but they do not commonly comprise a large proportion of the central Texas oak savannahs.

Diagnostic Protocol

Few other live oak diseases resemble oak wilt. Herbicide damage may mimic the foliar symptoms and patterns of mortality, but herbicides are not species-specific and can usually be distinguished from oak wilt with little effort. Absolute certainty is necessary in diagnosing any destructive tree disease, especially when expensive management programs are to be implemented. A five step diagnostic protocol is recommended for routine tree diagnostics in central Texas. Although designed to distinguish oak wilt from other diseases, the protocol has proven useful in diagnosing any tree problem. The completion of all five steps is unnecessary to make an accurate diagnosis and may be impossible. The five steps are: 1) observe the pattern of dieback and mortality in time and space for the tree population, 2) observe the patterns of dieback and mortality in time and space for individual tree crowns, 3) examine specific organs, e.g. leaves, branches, or roots, for unique patterns of chlorosis and/or necrosis (symptoms), 4) examine specific organs for evidence of the pathogen (signs), and 5) attempt to obtain laboratory isolation of suspected pathogens. An analysis of these five steps also provides an excellent tool for better understanding oak wilt. A brief description is provided below for each of the steps.

Pattern of tree mortality at the population level

An expanding patch of dead and dying live oaks spreading from diseased trees to adjacent, healthy trees on the perimeter is a familiar characteristic of oak wilt in Texas. Sequential, aerial surveys of the disease in large, homogenous live oak stands in rangelands have been useful for estimating fungal spread (8). Typical expansion rates of 14 - 16 meters/yr. were recorded. The fungus is able to spread similarly in urban trees, in spite of the expected obstructions resulting from construction in the native stands. In Figure 2, oak mortality can be followed over a 5 year period for a disease center in an Austin, TX, neighborhood designated as Mesa I. The initial infection probably occurred at the northern corner of the block in 1981. Tree crowns in Figure 2 are comprised only of live oak. The urban infrastructure for Mesa I is also depicted, showing how C. fagacearum may spread under sidewalks, driveways, construction slabs and sewer lines. The average rates of spread, sizes, and relative densities of live oak for Mesa I and other disease centers are given in Table 1. Relative density refers to total crown cover divided by total area of the focus in 1987. The relative densities for all four disease centers were comparable, but the rate of spread observed in Mesa I was the least of the four. Urban development may influence the occurrence of root grafting and common root systems and in turn reduce opportunities for root transmission. The mortality in Mesa I expanded to affect 12 property owners, comprising 3,280 m² of crown cover and 1.26 ha before steps were taken to minimize further losses.

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Figure 2.

Photo interpretation of an oak wilt center (MESA I) in Austin, TX, made from 1:24,000 scale cole-infrared aerial photography. Tree crowns, houses, streets, and locations of underground utilities are depicted in the drawing. Expansion of the disease center can be seen during the years 1982-1987.

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Table 1.

Rates of spread for Ceratocystis fagacearum, sizes, and relative live oak densities^x for two urban and two rural oak wilt centers in Central Texas.

In Texas, the application of soil sterilants is a popular method for controlling weeds in the landscape. As is the case with foliar symptoms, the effects of these compounds are sometimes confused with oak wilt because they may result in expanding patches of tree mortality. Eventually the patch ceases to expand, in contrast to the inevitable spread of C. fagacearum into adjacent live oaks on the focus perimeter. Other than herbicides, there are no known disease-causing agents that resemble the typical oak wilt pattern in a live oak population.

Options For Disease Control

The rapid rate of fungal transmission and tree mortality in Mesa I (Table 1) illustrate the need for prompt diagnosis while oak wilt centers are small and relatively easy to manage. Most oak wilt management practices are preventative; the selection of a suitable practice depends on whether the goal is to prevent overland transmission or local spread through root systems. The affected oak species, topography, tree density, and tree values also must be considered. For example, when oak wilt is being controlled in a stand of live oaks, control efforts can focus on root transmission due to the lack of fungal mat formation and insect transmission. This is best accomplished by creating a “barrier zone” around the perimeter of the disease center. The zone is established by digging a trench to break root connections at a minimum of 100 ft. beyond the outermost symptomatic live oaks located on the perimeter of the disease center. This was the reasoning used for placement of the trench in Mesa I (Figure 2) along Graystone Dr. and Rockpoint Dr. to prevent further spread to the south and southwest. The greater width of Mesa Dr. and recent placement of relatively deep water lines were considered to be sufficient to protect neighborhoods to the north. There are no oaks to the east at risk of root transmission. Trenching should always be to a minimum of 48 in. deep, but greater depths are preferable (Figure 3). Symptomless trees will exist within the barrier zone, some of which are infected but not yet responding. The best approach is to rogue these trees, including stumps, with a bulldozer to further break root connections, but this measure is rarely followed in urban environments due to the site disruption and high tree values. Additionally, several secondary trenches may be dug within the barrier to isolate the diseased, symptomless trees while saving the unidentifiable healthy trees.

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Figure 3.

Trenching to break root connections for oak wilt control in the urban environment.

In urban environments, barrier trees are usually sufficiently valuable to consider treatment by intravascular injection with the fungicide Alamo®. Alamo® (propiconazole) is an ergosterol-biosynthesis- inhibiting fungicide with several qualities that make it ideal for injecting trees to control vascular- inhabiting pathogens (9,10,22,29). In addition to being readily systemic with low phytotoxicity, propiconazole also has growth regulator properties that may promote disease control through an influence on tree physiology (11). Research conducted during 1987 -1990 demonstrated propiconazole was very effective as a preventative treatment and also had some therapeutic value for live oaks with oak wilt (2,28). A total of 100, native-grown live oaks were involved in the study to compare disease development between treated and untreated trees under natural infection (Table 2). Treated and untreated controls were grouped in 9 plots located in urban and rural oak wilt centers. The plots consisted of groups of trees in yards, or rural stands, measuring no more than 100 ft. across. Six of the plots were complete treatments (no. 3,4,5,6,8, and 9), and 3 were split plots (1A and B, 2A and B, and 7A and B). In split plots, treated trees were intermingled with untreated trees. All trees were located on the perimeters of disease centers, and some plots contained both preventative and therapeutic treatments.

A standard high volume, low pressure (20 -25 psi), root-flare injection technique was used to inject propiconazole into the trees (Figure 4). As the pathogen colonized root systems and symptoms developed, trees were rated according percentage crown loss from 9 - 36 months following treatment. Survival and crown retention was consistently greater in treated trees than untreated trees (Table 2). The mortality rate in the 57 treated trees was 8% in contrast to 70% in untreated trees. Crown loss ranged from none (Plots 3 and 6) to 41 % (Plot 5) in injected trees, compared to a range of 61% (Plot 2B) to 100% (Plot 9) in the untreated controls. Based on these results, propiconazole was labelled under the trade name Alamo® in 1991. Since then, injection has become a routine practice for homeowners and commercial arborists involved in the control of oak wilt.

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Figure 4.

Root flare, high volume injection with propiconazole for oak wilt control in live oak.

Although the research results justify recommending injection, the study revealed many limitations to using propiconazole for disease control. For example, injection does not control the spread of the pathogen through a stand. Injected trees appear to sustain root colonization and transmit the pathogen to other connected trees, although there may be little or no crown loss. Also, preventative treatments were more effective than treating symptomatic trees, although there was some efficacy to therapeutic treatments (2). In Texas, live oaks adjacent to symptomatic trees are considered to be diseased even though they may have no symptoms, due to the likelihood of latent root infections. These limitations, combined with other drawbacks inherent in the injection process (21,27), must be considered when formulating oak wilt control programs. Injection should be just one step in a total program to control the pathogen and reduce losses of trees.

When red oaks are diseased, there are several measures available to prevent overland spread by insect vectors. The safest approach is to remove and destroy all diseased red oaks immediately upon detection. Mat formation is then eliminated as an inoculum source. Nitidulid transmission can also be prevented by avoiding wounds on trunks, limbs and branches of healthy trees, especially during the spring, Fresh wounds attract nitidulids during feeding and if contaminated, the beetles will inoculate the wound (19,20). Wound paints can also be used to prevent the beetle from coming in contact with the exposed sapwood (14,15). The wound paints need only to protect the wound for 2 - 3 days, after which the wound is no longer susceptible to infection.

In Texas, the risk of infection by nitidulids is considered to be greatest during March 1 through June 1. Free-flying nitidulid populations are highest during this period. At the same time, peaks in mat formation occur on diseased red oaks. Finally, live oaks are most susceptible to infection during the spring (6,7). These lines of evidence, combined with research conducted in other states, justify an active campaign to encourage the use of wound paints and minimize pruning of trees during the spring.

Long distance overland transmission may also be prevented by avoiding the transport of firewood. Of greatest concern is the formation of fungal mats on logs cut from diseased red oaks. However, the possibility exists that contaminated nitidulid beetles emerging form fungal mats may be harbored on live oak logs removed from disease centers. Therefore, all oak firewood originating from areas with oak wilt epidemics should be treated with caution. All of the wood should be burned prior to spring, and covering wood piles with clear plastic to prevent emergence of insects is warranted.

Future Considerations

There are many facets of oak wilt requiring further research in order to better control the disease. The nature of disease resistance in the genus Quercus has not been thoroughly investigated, so that selection and breeding for improved trees is impaired. There are undoubtedly gaps in our information on potential vectors such as oak bark beetles, especially in Texas where there has been little work on insects other than nitidulids. As a result, the sources of localized disease outbreaks in previously unaffected areas are occasionally impossible to explain. On a broader scale, the origins of the pathogen in the U.S. are unknown, making it difficult to predict where and when C. fagacearum will ultimately spread and reach epidemic proportions. In addition to the continued expansion of the oak wilt epidemic into new parts of Texas, the disease may yet spread into western and Pacific Coast states where several Quercus spp. comprise important components of forests. During the developing oak wilt epidemic in Texas, C. fagacearum has proven extremely adaptable to environmental conditions, such as a hot climate, previously considered to be limiting to disease development. Just as methods to control oak wilt in the Mid-Atlantic and North Central States has limitations for use in Texas, new approaches will probably be needed as the range of the fungus continues to expand.

The successes achieved in minimizing oak wilt losses in Texas can be attributed in part to the cooperation of private arborists with several different municipal, state and federal agencies. Many individuals and institutions have shared time and resources toward conducting surveys, research projects and public educational efforts throughout a 50 county region in central Texas. Public understanding of the disease and the technical basis for control is therefor excellent. The USDA sponsored, Texas Forest Service Oak Wilt Suppression project provides homeowner-assistance in designing and implementing disease control strategies. County agents and extension specialists with the Texas Agricultural Extension Service have made oak wilt a high priority for public education workshops and programs to enhance the decision-making process needed to control the disease. The City of Austin, Parks and Recreation Department has played a key role in testing and demonstrating options for disease control in urban settings. These cooperative efforts have provided a model for coping with a destructive, urban tree epidemic while illustrating to the public the value of good tree maintenance practices.