”Today, for some, a universe will vanish…the silence of after.” Jane Hirshfield, Today, Another Universe

Too many of our oaks at Pleasant Valley are in decline. Some are dead, and standing; the storm on May 21 felled others. Trees continue to drop branches as rains and wind batter the area. What causes oak decline?

Michael Hillstrom, a forest pathologist with the Wisconsin DNR, toured the conservancy on August 5, 2024, at Kathie’s request. He said that oak mortality is widespread across the midwest, especially with older (>100 years) bur oaks. Age alone makes them more prone to decline, but drought/flood cycles, storm and frost damage, thin soils, soil compaction, excess heat, and high humidity all act as stressors that make trees more susceptible to disease and insect pests. A stressed oak is not as able to fight off opportunistic insect and fungal invasions. Its chemical weapons are compromised. And it is not just bur, but also white, pin, red and black oaks that are in trouble.

Causes of oak decline [I recommend you find and watch the spring 2024 webinar on oak decline given by Rachael Dube (Minnesota DNR) in conjunction with the Minnesota Sustainable Forest Education Cooperative.]

Bur Oak Blight (BOB) We’ve reported and described cases of but oak blight, Tubakia iowensis (a fungal disease), as early as October, 2017 (Tom’s blog). This disease is relatively new to the Midwest (1990s). Leaf symptoms start in the lower crown; the upper crown can remain normal for years. Leaf veins become darkened, and black fruiting bodies form at the base of the leaf petiole. Spore formation is favored by more spring and summer rainfall, greater humidity, and higher nighttime temperatures. Repeated leaf damage weakens these oaks and makes them susceptible, in addition, to other common pests.

Twolined Chestnut Borer (TLCB) Michael Hillstrom thinks that our stressed oaks are being attacked by the twolined chestnut borer beetle (Agrilus bilineatus). He said that TLCB has been the most common culprit for oak mortality in the Midwest over the last few years. Even so, trees can have more than one pathogen at the same time.

According to DNR fact sheets, TLCB is a native beetle that attacks weakened oaks—again, weakened by drought, wounding, flood, and other notable pests such as BOB, spongy moth, tent caterpillers, and Armillaria fungus (more on the latter later).

Adult twolined chestnut beetles are less than a half inch long and are glossy black with two light-yellow wing stripes. Eggs laid in late summer hatch cream-colored larvae that bore through the bark and begin feeding on the tree’s growth layers or cambium. In the process, they create an extensive telltale network of racetrack traces or winding tunnels (collectively a “gallery”), often laden with sawdust. The parts of the tree above the larval invasion begin to die. Larvae overwinter and emerge the next summer as adults to repeat the cycle.

TCLB attacks start in the upper portion of the tree, so the upper branches die first. Later, the mid-layer branches turn red or brown, while the lower branches remain beetle free and green for a relatively longer period of time. These patterns can be diagnostic. Trees that have lost many smaller branches may begin to leaf out along the major remaining limbs and trunk—what is called epicormic branching. A tree may take several years to die.

If the stressors lessen, chestnut borer populations decline, and trees may recover. That’s also good news for the healthier trees nearby that have so far managed to avoid becoming infected.

Armillaria (honey fungus) Armillaria is a native fungus that lives on trees and woody shrubs. According to Wikipedia, the mushrooms are edible (prized in Europe, Ukraine, and elsewhere), but should be cooked thoroughly and not consumed with alcohol. Armillaria species are long-lived, and one individual of A. ostoyae in Oregon is not only the largest living fungus in the world (>3.4 square miles), it is also 2,500 years old!

Armillaria causes “white rot” root disease in forests. An infected tree dies if it is girdled by the fungus or if the roots are heavily degraded. Dieback and leaf paucity are common in the spring. Layers of light-colored mycelia beneath the bark at the base of the trunk are characteristic, as are ropy strands called “shoestring rot” (basic information provided by Wikipedia). Armillaria is common in forests. Very healthy trees not under great stress usually have no problem living with or near it (Rachael Dube).

Phytophthora (water mold) Phytophthora is a fungal-like root rot that manifests as many, many specialized host-specific species. It has been an agricultural nightmare forever. This pathogen was responsible for the Irish potato famine (P. infestans), is famous for many food crop diseases (over 60 plant genera), and can cause sudden oak death (P. ramorum and P. quercia). Picture a root with fuzzy white stuff all over it. This genus is more closely related to plants than fungi are, and has a distinct evolutionary history. (Wikipedia informed)

The future of our oaks

Michael Hillstrom hopes that our healthy oaks, which are in the majority, will be able to fight off twolined chestnut borer and Armillaria if rainfall amounts continue to be normal. Even our infected trees, if still mostly healthy, may recover to some extent. He thinks we will see mortality end in a year or two, and suggests we protect our bur oak seedlings from deer (with fencing), and keep young oaks from fire damage. Other DNR experts say that trees with 50% dieback probably will never recover. Optimistically, woodpeckers likely eat the larvae, and dead tree snags make excellent wildlife habitat.

Needless to say, forest-wide pesticide use is impractical and ill-advised. Both TLCB and Armillaria are native to the area and are everywhere. If you have an old infected oak in your yard, systemic treatment may help if given early—and it will be expensive.

Afterthoughts

A severe drought year, such as we had in 2012 for example, stresses trees in many ways. Roots shrink, growth slows—pathogens move in. This can begin a slow, years-long decline, especially if major stressors repeat (a too-wet year, then dry 2021/23, for example). After a time, we notice.

And when a tree is gone…

…as Jane Hirshfield continues in her poem, Today, Another Universe:

”This hole…

the puzzled birds…”

…and, I add: the bereft bee and I will circle this emptiness and see only, at best, an intricate afterimage.