Talk:Oak wilt

WAGING WAR ON OAK WILT

Article Written by: Maggie Ambrosino, Certified Arborist Owner of Brown and Green Tree Consultants, Austin, Texas

Skeletons of dead oaks, standing gruesome and gray, are common sightings along the landscape and, closer to home, our neighborhood streets and parks. These images are far too haunting to ignore their existence. Many of our once grand oaks are now canopies of sparse and riddled decline, existing to the point of asset vs. liability—and many of the dead are left standing in testament to a dreadful disease too long ignored. Our live oaks are worth saving. Our live oaks are worth protecting. What other trees grace us with their 100-foot canopies of cool, sweet shade, and waylay against our harsh eco system to showcase their majesty for generations of beauty and enjoyment? The battle against oak wilt is far from over, and the fungal disease continues to progress approximately 80 feet per year. A contaminated oak can infect healthy adjacent oaks underground up to 200 feet away. Knowing what we now know, or should know, 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 by invading and disabling the water-conducting vessels in vulnerable trees—it is evident that pruning oaks should be done with knowledge and vigilance. We also know that oak wilt is spread two ways: by insect vectors that land on fresh cuts or wounds, and through grafted root systems. By acting on what we do know, with due diligence, we can make great strides against the disease. How do we recognize oak wilt? Look for the telltale signs evidenced by leaf symptoms, patterns of spreading decline, tip die-back, rate of tree mortality and close proximity of other oak wilt centers. Out-of-season browning, leaf drop and veinal necrosis are also warning signs, but words of caution please: Obtain a professional diagnosis. These symptoms are not all strictly indicative of oak wilt, and oak wilt is not strictly limited to these symptoms. How do we further manage oak wilt? First and foremost, make wise and informed choices on how we prune our trees. The daily buzz of chain saws can seem more like open season than pruning season. When choosing an arborist, make sure they are certified. Ask for that ISA card (International Society of Arboriculture) and ask for insurance! The inability to prove-up such requests should cast doubt. Drive-thru businesses leaving door-hangers and pulling trailers are not always your certified best bet, and though their labor costs are attractive, the value of resistance has a payback in the long run. It is better to save one tree at a time, if that’s all the budget allows, rather than accept that rock-bottom price for substandard work and risk spreading the disease. Ensure all tools are sterilized. Do it between every tree. Use pruning sealant on those oaks—on every single wound—whether it is from a saw or weed-eater or lawn mower scuffs across a lateral surface root. Vectors, which transport the disease from tree to tree, love the fruity smell of sap oozing from a fresh wound. Pruning paint or sealant helps to mask that smell. Prune live tissue from July to the end of January or in the hottest of summer and coldest of winter. Avoid pruning in the spring when the insect population is high and roots are vigorous. The bulk of the infection is transferred below ground, through grafted roots. It is the healthy oaks, more so than the contaminated oaks, which warrant our attention and protection, and a tree that still retains at least 30% of its canopy deserves any and all attempts to restore its health and vigor. How do we engage in direct battle with oak wilt? We can fight toward its prevention and suppression. The key to success with oak wilt, as with any disease, is recognition and swift response. Have those valued oaks injected by a professional, licensed applicator with an oak wilt-specific fungicide, or propiconazole, and avoid ineffective broad-spectrum fungicides; trench at a sufficient width and depth to sever root systems in areas where streets or utility work has not already done the job for you; and remove, wrap, dispose of or burn dead and dying red oaks immediately and never store the wood for firewood. The white oak family, which includes live oaks, is vulnerable to the red oak family which produces the deadly fungal mats responsible for the spread of the disease. White oaks can show symptoms over many years. The red oaks die quickly and suddenly, harboring lethal fungal mats beneath the bark. If cut wood 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. Lastly, if it’s oaks you love, consider diversifying your shade tree plantings to a more oak wilt resistant variety such as the burr oak, Monterrey or Mexican white oak, or chinquapin oak. There are also other natives and adapted species that are well worth considering. Unfortunately there is no complete cure for oak wilt. But there is a high rate of success in suppressing the disease and prolonging the life and beauty of our oaks for many more years to come if we remain as aggressive as the disease and, like our mighty oaks, hold our ground. For more information you may contact Maggie Ambrosino, 922-4649 or tjambro@sbcglobal.net, the Texas Forest Service, or check out these websites: www.isa-arbor.com/ www.texasoakwilt.org www.ci.austin.tx.us/oakwilt/ www.treetalkusa.com/

July 1, 2011 Windstorm in Minnesota and Wisconsin

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In 2012 and 2013, oak wilt disease showed-up in areas of east central Minnesota and northern Wisconsin where it had never been seen before. The infections appeared to have coincided-with the July 1, 2011 mesoscale convective vortex (MCV) windstorm, which had windspeeds of over 100 mph, because foresters found that first oaks infected were 7-1-11 storm-damaged oaks. They also knew infections happened within 48 hours of storm damage. West-facing slopes on the east bank (Wisconsin side) and terrace crests in the St. Croix river valley were hit hard with wind damage and oak wilt infections. Hilltops and bottoms and field edges, where oaks were more exposed, also had more serious wind and oak wilt damage. Approximately 3/4 million acres in Minnesota, possibly 10 times that area for Wisconsin and up to the Keweenaw Peninsula in Michigan, were affected by the storm. Millions of freshly wounded oak trees were now open to infection from trillions of oak wilt spores carried-in by the storm.

Infection Levels-Spore Loads

It's difficult for oaks to get infected in mid summer - after July 1 - because it's too hot for red oaks to sporulate and latewood cell development has been underway since early to mid-June. Two factors changed this around on July 1, 2011: the trillions of oak wilt spores carried-in from west-southwest and the windstorm's tearing/shredding/breaking-apart trees which re-opened oaks' 2011 & 2010 earlywood cells (earlywood is also called springwood, and latewood goes by the name summerwood in some areas). The oak wilt fungus is too big to fit within the smaller latewood cells, but can fit within and move upward through the much larger earlywood cells. This storm damage increased the chances of infection 1,000 - 1,000,000 times the usual. Instead of one new infection per 1,500 acres, foresters were seeing one new infection per 10 acres. The manageable level of oak wilt disease is one infection per 640 acres. One year after the storm, in summer 2012, oak wilt specialists found 10 - 20 infections per 100 acres both in Rushseba Township, Chisago County Minnesota and at St. Croix State Park in Pine County, Minnesota. Wounding & infecting at ten oaks per 100 acres, one oak per 100 acres or one oak per 200 acres presents a serious threat to the oak resource. St. Croix State Park's northernmost infected oak and southernmost infected oak were 10 miles apart. This broad coverage of oak wilt blanketing millions of acres is achieved only with huge spore supplies and re-opening of that years' earlywood cells.

Detection

The use of spectroscopy and spectroscopic imaging show promise for accurate detection of the disease and are currently being tested for large scale detection and monitoring.

Sources

Sources with that many hundreds of acres of oak wilt spores were upwind (240 degrees/west southwest) in the three oak wilt "reservoirs" on state and federal lands in Sherburne, Anoka, Isanti and Chisago counties of Minnesota (see Hosts and Symptoms). The sources of oak wilt spores were not nearby.

Rate of Spread

Before the windstorm, oak wilt could spread north at three quarters of a mile per year and east or west at one and one half miles per year. During the storm, oak wilt spread at up to 100 miles per hour. In the two years following the 2011 storm, infections were found at new locations in four Minnesota and at least one (Sawyer) of three Wisconsin counties. These 2011 infections have serious ecological/economic impacts, including loss of stumpage and real estate values, and will only get worse if summer (aerial with ground follow up) surveys aren't established as soon as possible.

Root Graft or Overland Infection?

The first oak in any infection center is always infected from overland spread; additional oaks killed from the disease in that infection center are killed from underground root graft spread. Before control plans are made, the landowner or land manager (forester, arborist) needs to find out how the infection got there and what type (underground, overland or both) it is. Root graft infections move slowly, so additional root graft infections may not be seen for 1–3 years. It is impossible for one oak infected via overland to infect & wilt seventeen additional oaks via root graft transmission, in less than 4 months (see Kerrick, Minnesota oak wilt photo, below). If one sees more than half dozen oaks wilting during one summer in a new infection with healthy oaks in between means infections started from overland route/storm event, not underground. In that case, each wilting oak is a separate infection center, not part of one infection center. One should also look at the site to confirm whether or not it's an overland infection. Overland infections occur on or near hilltops or other exposed areas, where taller oaks get wounded, then infected, and shorter oaks look healthy. A spread pattern with no obvious direction usually means multiple airborne/overland infections. Root graft infections can "fan" out in 360 degree pattern, but tend to head faster in one or two directions because of soil and topography changes and tree species arrangement/diversity. A birds-eye view of an infection center, during the middle of summer, going from oldest to newest based on colors would show: stone dead gray, brown, yellow, orange, green. So, if one doesn't see these color sequences or, a hub and spoke pattern radiating out, they are probably looking at multiple airborne infections. Looks can be deceiving because 2-6 large wilting oaks may often have only 1-2 trees left nearby to infect. Compared-to one oak deep in the woods that is just starting to wilt that can lead-to 2-5 dozen more nearby oaks dying in the next 5 years. So, proper ground ID & diagnosis is crucial for stopping this disease. Stopping spread with multiple infections means plowlines will be further out, so natural barriers should be located first. In the Lake States, a vibratory plow line is installed 54 to 72 inches deep in the ground and 60 – 150 feet away from the diseased oak. Plowing done correctly ruptures healthy roots that could, in the future, transmit oak wilt to neighboring healthy trees. Installing plow lines far enough away from diseased oaks is key to preventing root graft disease transmission. In southern states like Texas, where heavy clay or Caliche soils are prevalent, rocksaws are commonly used in lieu of vibratory plows. Infected oaks should not be cut down before vibratory plowing because, doing so will speed up oak wilt root graft spread to nearby healthy oaks. Base of trunk or root flare injections of azole or copper based fungicides can help contain foliar - or top down - infections but are costly and have less success containing root graft - bottom up - transmission of oak wilt.

Strategies - Containment vs. Control

There is a big difference between containment and control. Containment happens within a few hours or days; control takes weeks or many months. The lack of appreciation and understanding for disease control methods and strategies has led to a lot of landowners and taxpayers thinking there is no way to control oak wilt. Despite 40–50 years of work & study showing oak wilt as one of the easiest to control forest diseases in North America. Putting a plowline around an oak wilt infection center is containment, which is not controlling the disease. Going inside the containment lines or natural barriers/edges/cover type changes and making sure all oak wilt infections are dead and gone is control. Landowners and managers have to ask themselves this question (are we going to be containing or controlling this disease?) before starting a disease control project, whether in a small yard or on hundreds of acres. Control also means removal and processing - mostly by logging - of future spore-producing oaks. For the last 40–50 years, experienced professionals throughout North America have been doing insect & disease control year round, not four or six months per year. They stop oak wilt spread by identifying future spore producing red oaks, which trees to protect from wounding, which trees not threatened by root graft infection then, respond same day to tree health changes. Agriculture and construction work starts in April–May each year, so it behooves oak wilt specialists to continue disease control work and prevention right through spring and early summer. After 2005, treatment goals shifted towards "passive oak wilt control" on properties less than 5 acres and treating to natural barriers or edges for large acreages. Passive control means letting the disease burn out over a few years which allows other native tree and shrub species to take over. Treating to natural barriers means restoring to native forest or savanna species by harvesting out to where oak wilt can no longer expand via root grafts. All oaks of the species that is infected are logged out, whether they are infected or healthy. Passive oak wilt control on large acreages creates short and long-term problems for oaks. White oak group trees are at risk for infection from nearby sporulating red oak group trees for the short term; invasive species can take over and monocultures of oaks can re-develop that sustain oak wilt infections for 50-100 more years making for long term risks. Treating to natural barriers/edges or cover type changes is by far the best method to use when controlling oak wilt on large acreages.