Deep Root Zone Affects Probability of Oak Wilt Breakouts in Central Texas

Oak wilt is one of the most devastating tree diseases in the Eastern half of the United States, severely impacting oak trees (Quercus spp.) (Juzwik et al. 2011). Since its first detection in Texas in 1961, it has been found in 76 counties throughout the state (Dooling 1961; Texas A&M Forest Service 2025). Belowground spread of the pathogen (Bretziella fagacearum) is possible due to root grafts between oaks (Juzwik et al. 2010; Koch et al. 2010). Interconnected roots of live oaks (Q. fusiformis, Q. virginiana) play a critical role in the belowground spread of the pathogen across central Texas (Appel 1995). This means of pathogen spread results in large, rapidly expanding centers of mortality and is considered to be responsible for the majority of tree losses.

Integrated pest management approaches for oak wilt include rapid detection of new oak wilt infections followed by prompt root graft severing, tree removal, and/or root flare systemic injections with propiconazole (Appel et al. 1990; Billings et al. 2001; Blaedow et al. 2010; Koch et al. 2010). In some cases, girdling and herbicide treatment is used as a less resource intensive alternative to graft severing and tree removal (Bronson et al. 2023). Creating trenches, based on severing root connections, is recommended to limit the expansion of those disease centers by stopping the spread between the root systems of diseased and healthy trees (Himelick and Fox 1961). The success rate of trenching ranges from 67% to 88% in Texas (Appel 2001; Billings et al. 2001; Koch et al. 2010).

Although trenching is an effective method, its implementation can pose economic challenges for landowners (Billings 2008). Through the Texas A&M Forest Service Oak Wilt Suppression Project, landowners may apply to cost-shares for trenching, reducing the economic cost of the management strategy (Billings 2008). More than $2.9 million of federal cost-shares have been delivered to participating landowners since 1988 as an incentive to treat expanding oak wilt centers. To date, the Suppression Project has installed more than 3.6 million ft (680 mi) (1,094 km) of trenches to control 3,118 oak wilt centers. As part of the project, post-suppression evaluations are conducted to evaluate the effectiveness of trenches 5 to 7 years following installation.

Few studies have looked at the influence of soils on the distribution and management of oak wilt in Northern states. Bruhn et al. (1991) observed that belowground spread took place more frequently in sandier soils in Michigan and among oaks with larger diameters. Gearman and Blinnikov (2019) observed a significant relationship between B. fagacearum incidence and 2 specific soil orders in Minnesota: sandy riverine soils and fibrous, high-organic-matter wetland soils. More recently, Stevens et al. (2023) showed that spatial dependence, the distribution of oak forests, and soil bulk density had a significant role in the distribution of oak wilt in Wisconsin.

Despite these studies having looked at the influence of soil factors influencing pathogen transmission in Northern states, no studies have been conducted in the Southern distribution of oak wilt. Moreover, soil factors related to the success or failure of trenching to reduce the spread of oak wilt in Texas are unknown. Here we studied the effect of root zone depth and particle size on the probability of trench breakouts in central Texas.

A total of 2,124 trenches were assessed between 1990 and 2020, yielding an efficacy rate of 81% (1,727 trenches were deemed successful). The analysis was based on 286 georeferenced post-suppression evaluations that met the required criteria (trench depth, evaluation date, field confirmation, etc.), with 111 instances (38%) recording a breakout. A summary of breakouts, categorized by particle size and root zone depth, is presented in Table 1.

The probability of having an oak wilt breakout was significantly predicted by the simple effect of root zone depth and particle size; the estimated parameters (logit scale) are shown in Table 2. Regardless of soil particle size, chances of a breakout increased with root zone depth (Figure 1), with 26% probability at 20 cm to 80% probability at 200 cm on average for all particle sizes. Regarding particle size, clayey particle size had the highest probability of breakout (58%), and fine and fine-silty soil types had a significative lower probability than clayey on breakouts. Clayey-skeletal soils had a 49% probability to breakout, followed by fine-loamy (44%), loamy-skeletal (42%), loamy (39%), fine soils (18%), and fine-silty (4%), respectively. The model explained 4.6% total deviance or observed variation. No tendency or correlation was found on the residuals.

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Figure 1.

Predicted probabilities of breakout for different root zone depth (cm) and soil particle size. Shaded areas represent confidence intervals around the predicted values.

Trenching for oak wilt management aims to prevent belowground spread by severing root connections between infected and healthy oak trees. Inadequate depth or poor placement of the trench often leads to treatment failure (Juzwik et al. 2011). Successful trenching relies on regional equipment availability and cost, and the placement of the primary line must extend beyond the radius of infected trees to be effective (Koch et al. 2010; Juzwik et al. 2011). Underground spread of B. fagacearum leads to expansion of existing infection centers, affected by the distance between infected and susceptible trees, root graft frequency, oak species composition, and environmental conditions, especially soil type.

Our results suggest that root zone depth and particle size are relevant variables in trench failure in Texas. All soil particle sizes showed the same slope of root zone depth, with lower probability of breakouts in shallower rooting depth. Following cost-shared trench guidelines, trenches are standardized to a depth of at least 122 cm (4 ft). Because chances of breakout increase with root zone depth, areas with deeper soil should be carefully evaluated to make sure that root connections are being severed. Our study suggests that even when trenches are installed in a low root zone depth, lower probability of breakout is expected depending on soil type, with fine soils and fine-silty soils showing the lower probability for breakouts. On the other hand, clayey soils show higher chances of breakout. This pattern could be explained by the differences in vertical rooting patterns across soil types. Studies on Quercus sp. have shown that in clayey sites, a relatively large fraction of roots is found in deeper soil layers, whereas at silty sites, the rooting pattern was most superficial (Thomas 2000). Therefore, given that standard trenching guidelines recommend a depth of at least 122 cm (4 ft), our results underscore that this depth may be insufficient in areas with deeper root zones, particularly where the inherent soil properties, such as those found in clayey soils, promote more extensive vertical root development.

Because B. fagacearum does not interact, spread, or have any relationship with the soil, the results of this analysis do not imply a relationship between soil texture (percent of clay, loam, or sand) and pathogen spread but rather examines how soil type and texture may play into the root zone depth which could predict trenching success. However, water holding capacity can be affected by texture and might impact root growth and grafting due to differences in moisture availability. Clay-rich soils, for example, hold more moisture than sandy soils, with enhanced woody biomass and higher fertility (Kramer and Boyer 1995; Laurance et al. 1999). This may contribute to more extensive and deeper root system development, which, in turn, could explain the increased likelihood of trench breakouts observed in these soil types in our study, as these deeper connections are harder to sever effectively with standard trenching depths. Another key factor to consider is the formation of a shared root system in live oaks, which directly influences how the pathogen spreads across the landscape. To fully understand the variables involved, it is likely essential to distinguish between the biological processes of grafting and those of sprouting, along with the subsequent survival of the interconnected trees.

Our findings align with previous research indicating the significant influence of soil characteristics on oak wilt dynamics. Recent findings in oak wilt research in the Northeastern United States highlight the importance of considering soil and landscape features when developing management strategies. In Wisconsin, Stevens et al. (2024) suggested that oak wilt spreads more rapidly in sandy soils and flat terrain due to increased root grafting between trees. In contrast, clay-rich soils and uneven topography tend to slow the disease’s progression in that region. While Stevens et al. (2024) observed a faster spread in sandy soils in Wisconsin due to increased root grafting, our findings in central Texas suggest that the deeper rooting patterns characteristic of clayey soils may increase the probability of trench breakouts by allowing root connections to persist below the standard trenching depth, even if the overall disease progression rate might differ. Moreover, in Minnesota, land cover and soil type were important variables contributing to the prediction of the distribution of oak wilt (Gearman and Blinnikov 2019). While studies in Northern states like Michigan and Wisconsin have highlighted the role of sandy soils or specific soil orders in pathogen transmission, our study provides novel insights into how root zone depth and particle size, particularly in clayey soils, impact the success of trenching in the unique central Texas environment.

Our model explained 4.6% of the observed variation; besides root zone depth and particle size, there might be other relevant variables influencing trench success, including equipment used to conduct the trench, length of the trench, and history of oak wilt in the area that can be included in future analysis. Historic trenching data from post-suppression evaluations compiled by this study since the inception of the Oak Wilt Suppression Project suggests that trenching has an overall success of 81%. Evaluation of cost-shared trenches in the early 1990s suggested that equipment used (rocksaw vs. bulldozer with ripper bar vs. chain trencher), month of treatment, and land category might affect the breakout probability (Gehring 1995). Interestingly, the author reports similar findings with 80% of the treated centers holding without any breakouts. Additionally, more breakouts were observed in urban environments with rocksaws compared to rural environments with bulldozer with a ripper bar.

This study provides the first quantitative evaluation of trench efficacy in Texas, extending prior research concentrated in the Upper Midwest (Juzwik et al. 2011; Stevens et al. 2024). These findings offer critical insight for regional adaptations of oak wilt suppression protocols. Other variables are needed to understand not only trench failure but the spread of oak wilt in Texas. Spatial modeling including all mortality centers recorded through the Oak Wilt Suppression Project could be developed, adding additional factors such as weather, terrain, and soil properties. Because clayey soils are more conducive for breakout, trench installation and management could also be explored, including backfilled materials and depth of trenches for these types of soils. In addition, future studies can identify differences in spatial dynamics of B. fagacearum driven not only by forest community but by latitude, soil types, and management practices.

The authors reported no conflicts of interest.