Oaks make up 30% of our North American forests. MITPPC researchers are trying to save them from a deadly disease.
September 10, 2019
by Dana D'Amico and Caro Silvola
The word for the oak genus group, Quercus, is thought to originate from Celtic – “beautiful tree.” Ecologist Jeannine Cavender-Bares has spent the past 20 years studying Quercus, just one sixteenth of the lifetime of the oldest oak in Minneapolis that is now nearing 320 years of growth.
Oaks make up 30% of our North American forests. Recently, Cavender-Bares has noticed that they are dying.
One of the primary culprits is oak wilt, an invasive fungal disease that can be difficult to detect until it’s too late. It spreads easily between trees through connected root systems and insect carriers and is almost always fatal over time. The Minnesota Invasive Terrestrial Plants & Pests Center (MITPPC) places it as the second-highest ranked pathogen threat in the state, behind Dutch elm disease.
“It's not just an iconic group of trees at stake, it’s a third of our forests,” said Cavender-Bares. “It would be unbelievably tragic to lose oak trees. It does take effort to stop the progression of this disease, but it can be stopped. We just have to know where it is, and what it is, so that we can get a handle on it.”
Along with US Forest Service plant pathologist Jennifer Juzwik and University of Minnesota forest ecologist Rebecca Montgomery, Cavender-Bares is co-leading a team of researchers to fight oak wilt in Minnesota and beyond.
Shining new light on oak wilt
One of the key challenges of detecting oak wilt is how similar the symptomatic bronze or brown wilting leaves can look to signs of drought stress and other diseases.
Finding infected trees in a large plot of forest land can be like searching for a needle in a haystack, especially when wilting is in its more subtle early stages. Some managers do it on foot.
To make the detection process easier, the team has turned to a remote surveillance technique called hyperspectral imaging. It’s a big term for a familiar idea – using light to gather information and make inferences about what lies in the frame of view, the same way that our eyes do.
“Your eyes can see a vast array of things, but your eye is really working with three bands of energy or light,” explained Cavender-Bares. “It uses the integration of those three bands to see everything it sees. When we’re talking about hyperspectral wave reflectance, or spectroscopy, we’re talking about hundreds and hundreds of bands.”
A sensor used in hyperspectral imaging is a lot more sensitive than a human eye, and it can paint a much fuller story for researchers.
“We can learn quite a bit about why a plant might be sick based on how the light is interacting with it."
“Imagine everything your eye can see, and multiply that by hundreds,” she said. “There’s so much more information in all of that light energy to tell us about what might be going on, that we could never see with our eyes. We can learn quite a bit about why a plant might be sick based on how the light is interacting with it.”
For the past five years, Cavender-Bares has worked with NASA collaborators to obtain airborne images of Minnesota ecosystems. It was NASA’s pilots who flew over Cedar Creek forest sites to collect data for the oak wilt team in collaboration with colleagues at the University of Wisconsin and University of Nebraska.
Using hyperspectral image data, the team believes they can now identify oaks from other species with 95% accuracy, and the difference between healthy and sick trees with 84% accuracy.
Their next step, aimed at lowering cost, will be to test protocols that can use satellites or drones in lieu of manned flights for canopy detection.
A researcher holds a portable spectroradiometer, which is used for remote sensing of oak wilt in the field.
An oak wilt sapling is prepared for inoculation with oak wilt fungus as part of an experiment. (Credit: Caro Silvola, MITPPC)
A researcher drills into an experimental oak sapling before inoculating with oak wilt disease as part of an MITPPC project. (Credit: Caro Silvola)
An operator drives a vibratory plow, which is a machine used to cut grafted oak root systems in an effort to prevent or slow the underground spread of oak wilt disease. (Credit: Jennifer Juzwik)
Visible symptoms of oak wilt in an experimental oak sapling inoculated with fungus by MITPPC researchers. (Credit: Caro Silvola)
Containing a wildfire
As with any disease, diagnosis is only half the battle. Equally important is what it leads to: containment and control.
Much of the danger of oak wilt lies out of sight, in the soil. Because oak roots tend to connect below ground, it’s easy for the disease fungus to move between trees. Scientists estimate that up to 90% of the red oaks that will die of oak wilt each year in Minnesota are infected through grafted roots.
Jennifer Juzwik, a forest research plant pathologist with the US Forest Service and MITPPC project co-lead, is the team’s boots-on-the-ground expert on disease management. Like Cavender-Bares, she has decades of expertise in oak systems.
“Oak wilt is like a wildfire,” said Juzwik. “The disease can appear to be controlled based on what is seen above-ground, but it can continue ‘burning’ – spreading – below ground. It can pop up again several years down the line if it isn’t controlled and monitored.”
“Oak wilt is like a wildfire...it can pop up again several years down the line if it isn't controlled and monitored.”
One important tool used to halt underground transmission is a machine called the vibratory plow – a cable-laying plow retrofitted with a 60-inch blade that is used to cut shared root systems.
Plowing roots to stop transmission is nothing new, but information about best practices and success rates is not well documented. Historically, a single pass with the plow has been used to cut roots along a predetermined path to halt fungal transmission. Oak wilt foresters and scientists have noted re-grafting of roots across the single cut line as early as 5 years after this method.
A few plow operators have tried two passes – a double plow line – along the path in attempts to extend the time of treatment effectiveness. Juzwik feels there is an urgent need to evaluate and document the effectiveness of this modified approach through scientific means.
“For several decades, oak wilt foresters have used a vibratory plow to contain on a local site or parcel of land, oak wilt. We have documented single-pass treatments on heavier soils in hilly terrain, but we need to capture how effective the single-pass is on sandy soils in flat terrain, and then compare it to the double-pass system,” said Juzwik.
“We’re also evaluating the particular model for path placement used by our plow operator, who has done this for 40 years. I told him we want to document how he does it to teach the next generation. I think it’s important to document what an experienced forester does here in Minnesota.”
“I think it's important to document what an experienced forester does here in Minnesota.”
Researchers inoculate oak saplings with oak wilt disease fungus as part of their experiments. (Credit: Caro Silvola)