Waging war on oak wilt

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Article Written by: Maggie Ambrosino, Certified Arborist

Owner of Brown and Green Tree Care and Consulting Company, Austin, Texas
Skeletons of dead oaks, standing gruesome and gray, are common sightings along the landscape and, closer to home, neighborhood streets and parks. These images are far too haunting to ignore. Many of our once grand oaks are now canopies of sparse and riddled decline, existing to the point of asset vs. liability, left standing in testament to a dreadful disease. Our live oaks are worth protecting. Our live oaks are worth saving. In the hot of Central Texas, what other tree graces us with a one hundred-foot canopy of cool, sweet shade and showcases its majesty for generations of beauty, provision and enjoyment?

The battle against oak wilt is far from over, and this lethal fungal disease still continues to progress with economic destruction at an alarming rate. Research shows that a symptomatic oak can infect healthy oaks up to 150 feet away. A path of mortality can impact and devastate several acres or move through numerous contiguous properties in just one season. There is evidence that the summer heat of Central Texas tends to suppress the pathogen, having been the saving grace for many trees in the past, particularly native oaks. However, record-breaking heat and drought are cumulatively contributing profound stress in trees already weakened by oak wilt, robbing them of any chances of acquiring the excess vigor needed to win the race against the pathogen and survive.

Knowing what is now known, or should be known about oak wilt—that it is one of the most destructive tree diseases in the United States, and the lethal fungus, Ceratocystis fagacearum, spreads and kills in epidemic proportions through the water-conducting vessels in vulnerable trees—it is evident that preservation of oaks must and should be done with knowledge, vigilance and, this year in particular, lots of watering. This is a double-edge sword. Water contributes to pathogen mobility, prompting live oaks to produce a waxy substance called tyloses secreted to curtail movement of the pathogen. This substance curtails the flow of much needed water throughout the tree and instigates an early demise.

Research shows that oak wilt is spread two ways: 1) overland by insect vectors or sap-feeding beetles that transfer and deposit fungal spores from infected red oaks when, in turn, they feed on fresh cuts or wounds on healthy oaks and, 2) in the underground environment, through grafted root systems…live oak to live oak or in some cases, red oak to live oak. By acting on what is known, with due diligence, great strides can be made against the disease and being pro-active rather than re-active is a mightier strategy.

How is oak wilt recognized? Look for the telltale signs evidenced by leaf symptoms or veinal necrosis (haloing of veins), observing paths of mortality or spreading decline, red oaks failing to produce their canopies in spring or their sudden death, tip die-back, and the rate of mortality-particularly in red oaks-which, again, die very rapidly. Continue to monitor the proximity of other symptomatic oaks or oak wilt centers and discern out-of-season browning and leaf drop. Not every tree will display that exemplary leaf pattern. All of these symptoms are not indicative of oak wilt only as oak wilt is not the only tree malady defined by these symptoms, so it is important to obtain a professional diagnosis.

How is oak wilt avoided? Exercising wise and informed choices on how trees should be pruned and maintained are the better considerations in the decision-making process. The daily buzz of chain saws can seem more like open season rather than pruning season. Prune live tissue from mid-June to early February or, as a good rule of thumb, during the hottest and coldest time of year when insect populations are low and above-ground growth is slow. Avoid pruning in the spring unless necessary, and assess storm damage readily. When choosing an arborist, make sure they are certified. (See Why Hire an Arborist link on the web site.) Ask for that ISA card (International Society of Arboriculture) and ask for insurance! The inability to prove-up such requests should cast doubt. Some certified arborists are oak wilt certified as well which is a real plus. Drive-thru businesses knocking on doors or leaving door-hangers or landscape workers pulling trailers loaded down with mounds of live tissue are not always your best bet, and though their tempting labor costs seem attractive, the value of resisting such a lure has a huge payback in the long run. Remember, leaves are food or carbohydrates for the trees providing present and future resources. When trees are striped of live tissue due to lack of pruning knowledge, and when the oak wilt pathogen moves through that area and trees begin to lose their canopy, less food will equal less vigor and a faster demise. It would be best to prune one tree at a time using a trusted and knowledgeable professional, if that’s all the budget allows, rather than accepting that rock-bottom price for substandard work, risking the spread of disease.

Ensure that all tools are sterilized. It must be done between every tree. Use pruning sealant or spray paint on oaks—on every single wound no matter how small or high up, year round—whether it is from a saw or weed-eater or lawn mower scuffs across a lateral surface root. Vectors are attracted to the fruity smell of sap oozing from a fresh wound. In many parts of Central Texas there are visible insect populations all year long. Pruning paint or sealant simply helps to mask the smell of sap. That is the only purpose of painting wounds and live oaks are the only trees that need pruning paint—no other species. If all you have on hand is a can of spray paint in your garage, use it immediately in the case of wounding. This is not necessary if only dead wood is being pruned, but that can’t be guaranteed and it is always best to err on the side of caution. So, practice or demand wound painting and sterilization of tools.

The bulk of the infection is incurred and transferred below ground, through grafted roots. It is the healthy oaks, even more so than the infected oaks, which warrant our attention and protection, but a tree that still retains at least sixty to seventy percent of its canopy deserves any and all attempts to save it, though the fight for survival will be more of a challenge once symptomatic. A non-symptomatic tree, if inoculated, has about a ninety-seven percent survival rate. The success rate on a symptomatic tree plummets, no matter how healthy or large. Trees must be inoculated when they exist within 150-200 feet of an infected tree.

How is the battle against oak wilt won? Because there is no present cure, fighting for prevention is key. Success with oak wilt, as with any disease, is early diagnosis and swift response. Once vulnerable, have those valued oaks injected by a professional licensed applicator with the recommended oak wilt-specific fungicide—propiconazole, referred to as “Alamo,” and the “Macro” infusion method of treatment. Avoid any ad or vendor representing or pushing the use of topical or spray fungicides or soil treatments of any kind to treat oak wilt. This practice is fraudulent as no other organic or chemical application, other than those mentioned herein, are approved by the EPA for use in treating oak wilt. Tebuconazole, or the “Micro” injection method of treatment, has been known to be more effective in smaller trees or trees in hard to reach places, and some arborists use this method as it is less time consuming, less laborious and simple to use. Tebuconazole requires yearly boosters that prove expensive over time, and since micro injections rely on the trees’ own water conducting vessels and ample soil moisture to help carry the medicine, coverage is debatable particularly when there is minimal soil moisture and low moisture content in trees during drought or the hotter summer months. There is little to no product research available to either consumer or applicator although the product manufacturer, Mauget, is a highly reputable company which promotes a proven line of tree-care products administered via a patented delivery system.

Much research is available on Propiconazole or, “Alamo.” Depending on the individual tree and it’s successful uptake of the medicine, a second dose is not necessary for another year and one-half or so. A good arborist/applicator works closely with property owners to discuss retreatment based on the merits of current and future tree health and vigor. Arborists who do the job right, who excavate or air spade a true representation of the flare roots, who use proper dosing in relation to the percent of disease progression, who administer the proper number of injection sites, who monitor the equipment and tubing harnesses for leakage and who are not production companies but maintain a more personal, hands-on monitoring see high rates of success.

The option of quarantining and trenching to encompass and remove all infected trees is still the best battle against oak wilt in rural areas, but isolating and trenching around infected root systems or cutting down trees in front of residences is often impractical for most property owners in urban neighborhoods who prefer to keep their symptomatic trees rather than fell them and lose all associated value, and the cost is out of reach for most without a good amount of disposable income or pooling funds. Extreme caution must be taken to trench far enough out ahead of the advancing diseased roots (150-200 feet) at the proper depth (5-8 feet deep depending on soil conditions) to ensure absolute division and protection of uninfected trees from sick trees. If not properly calculated, it would be all for naught. Oftentimes the Forest Service will work with property owners to subsidize costs, if funds are available, particularly where trenching or removal of red oaks is concerned. You can also find a link to the Texas Forest Service web site located in the Texas Tree Info Site on this web site.

The white oak family is vulnerable to the red oak family which produces the deadly fungal mats responsible for the spread of the disease. Let’s all look up and around and take precautions with red oaks when they seem to just “up and die.” The Forest Service has a rule: If it’s dead and it’s red, remove it. Promptly remove, wrap, dispose of or burn dead and dying red oaks immediately, and never store the wood for firewood. With early intervention, white or live oaks can show symptoms over many years and still survive. Red oaks, too, are treatable but can only be saved if they are non-symptomatic at the time of treatment. The vascular system of a red oak is large in comparison to a live oak, and the pathogen moves through at a voracious rate, killing the tree so quickly the canopy is often left in full retention. If cut wood from a fallen red oak cannot be removed off-site immediately, it should be covered with a clear plastic tarp and the edges of the tarp buried below soil level. Live oak wood poses no threat.

Lastly, if it’s oaks you love, consider diversifying your shade tree plantings to a more oak wilt resistant variety such as the white oak, post oak, burr oak, Monterrey or Mexican white oak, lacey oak or chinquapin oak. Diversification of species within a mono-cultural setting is valuable advice so that all trees are not lost should a species-specific pathogen move through like the Dutch elm disease which was responsible for the massive economic loss of American Elms and property values in the North. (By the way, the DE disease is showing up in some areas of Texas). There are many natives and adapted species that are well worth considering so go on-line or spend a day at your favorite nursery or contact a certified arborist familiar to the area. See also the Forest Service’s recommended list of trees to be found on this web site.

In the end, even with our help, it is up to the individual tree and its health and vigor that will govern its survival. Also remember that without water, trees are much more vulnerable so not enough can be said for watering. If you do not have a sprinkler system, buy flow-through sprinklers, cut an old hose in five-foot increments, connect as many together as you want with male and female connections from hose repair kits and make your own. And if your soil is compacted, less water percolates down where it is desperately needed so aeration is important.

Unfortunately, there is no cure for oak wilt, but there is a high rate of success in suppressing the disease by early intervention, sound cultural decisions and maintenance practices. We can no longer afford to be vigilant with only our own trees, but continually observe those along our neighborhood streets and the general vicinity as well. We must remain as aggressive as the disease itself and, like the mighty oaks, hold our ground. For more information you may contact Maggie Ambrosino at Brown and Green Tree Care and Consulting Company, 512-922-4649, or send a quick note to brownandgreentreecare@yahoo.com.

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