The large tusks tear through the soil tossing bits of sand, rock and root skyward. Grits of sand grind between my clenched teeth; my taste fouled by sunscreen, sweat and blood. I feel my momentum waning, and strain hard to pull the reluctant chainsaw-like beast up the steep terrain. I hold on to the flailing brute, pulling with everything I have to break the stalemate. It’s not enough. My strength is gone.
A week earlier, we received a report from the UW Plant Disease Diagnostic Clinic that the tissue sample we submitted was confirmed to be oak wilt. According to the MN DNR, oak wilt is a deadly disease that affects all species of oaks (Quercus). It is caused by a nonnative, invasive fungus (Bretziella fagacearum). Oak tree species in the red oak group (leaves with pointed lobes) die about two months after infection. Bur oaks die between one and seven years after infection, while white oaks die from one to more than 20 years after infection. Some white oaks can recover from oak wilt.
The disease forms fungal mats on red oak species killed the previous year. The mats attract sap beetles, which spread the disease overland as they visit fresh wounds on live trees. Once a tree contracts oak wilt, the disease can spread to neighboring oak trees through root grafts.
Infected trees exhibit off-green, bronzed or wilting leaves on the outer canopy. Red oaks die one to two months after initial symptoms and drop most of their foliage. White and bur oaks will typically show only a portion of their outer canopy affected with off-colored leaves during a growing season. Abundant green-, brown- and bronzed-fallen leaves under an oak during the growing season signals oak wilt. If wilt starts in late summer, early fall, or during drought, oaks tend to maintain more dead leaves in their canopies.
Avoid pruning oaks during the growing season, especially April through July. There are many strategies utilized to control oak wilt such as severing roots, tree removal, herbicide treatment, stump removal and log burial. Most of these techniques are expensive and success depends on how prevalent oak wilt is in the local area.
It was nearly 20 years ago when I struggled with the walk-behind trencher on the steep hillside. Our hope was to isolate the infected tree from neighboring oaks by severing any root grafts between them. The tusks of the trencher only got us part of the way there, and I ended up finishing the steepest part with a hand spade.
Back then, I had a good back and optimism. While our efforts did work, a summer storm the following year inflicted widespread tree damage followed by several more outbreaks of oak wilt in other areas of our property. It was a lesson I’ve learned many times over the last 30 years – invasive species are best controlled at the landscape level rather than in small isolated pockets of control.
In recent years, another troubling oak disease has become widespread, bur oak blight (BOB). According to the MN DNR, BOB affects bur oak trees resulting in leaf browning and leaf loss in late summer and early fall. A native fungal pathogen called Tubakia iowensis causes the disease. Above-average rainfall for the past 30 years likely boosted the occurrence of this pathogen, leading to bur oak blight.
Bur oak blight’s early symptoms appear in midsummer, but the most obvious expression happens in late summer. Leaf symptoms include dark veins on the undersides of leaves and brown, wedge-shaped segments between leaf veins. The disease starts in the lower canopy and progresses up the tree. In severe cases, all but the outermost leaves around the canopy will die. Bur oak blight might cause minor dieback (death of branches starting at the tip), but it will not kill major limbs.
We have several gigantic bur oaks on our property that have had severe infections of BOB for many years now. Typically, starting in August, all but the uppermost canopy leaves fall to the ground. While long-term consequences of BOB are not known, weakened trees may eventually fall victim to invasion by opportunistic insects and fungi such as the two-lined chestnut borer and Armillaria root disease.
According to the UW Extension Insect Diagnostic Lab, the most important insect cause of oak mortality is the two-lined chestnut borer (Agrilus bilineatus). This insect is attracted to stressed and weakened oaks. Environmental extremes (e.g., drought), soil compaction, defoliation by leaf-feeding insects, storm damage and weakening from disease are all stresses that predispose trees to two-lined chestnut borer attack.
According to the UW Extension Forest Ecology and Management, Armillaria root disease, also known as shoestring root rot, is an often lethal disease of tree and shrub roots and lower stems. It can affect almost any conifer or hardwood species, from seedling to maturity. Trees and shrubs stressed due to drought or defoliation can be particularly susceptible to Armillaria root disease.
There are other diseases that affect oaks such as anthracnose, twig canker and leaf blister. A good introduction guide to help wade through all these is called “How to Recognize Common Diseases of Oaks in the Midwest: A Quick Guide,” by Jill D. Pokorny of the US Forest Service.
But the greatest threat to oak trees is the collapse of oak ecosystems. Across the upper Midwest, oak tree regeneration is almost nil. There are few recruitments to replace fallen, aged oaks. As this keystone species slips away, so too will the hundreds of species that depend on it. Competition from invasive species and changing land management practices are mostly to blame. Oak seedlings need lots of sunlight to survive. We no longer cultivate the landscape with fire as Native Americans once did. Woody invaders from other continents as well as non-fire-tolerant native woody forest species have choked our woodlots full, resulting in dense shade.
It’s painful to watch a titan fall. Many of these mighty, majestic oaks have borne witness to our history for well over 200 years. I find solace in knowing that ecological restoration practices offer a bright spot in the dim future of oak ecosystems.
On these “restored” lands, at least for now, young oaks are rising again. My hope, hundreds of years from now, some will become the titans that inspire humankind – a living bond between the past and present.