Oak decline, twolined chestnut borer and Armillaria


row of oak trees on a summer day
Following prolonged drought, this forest in Rum River State Forest started declining.


It can be a striking sight: pockets of red and brown canopies that stand out against an otherwise green forest. If you’ve noticed trees with dead or dying branches, unseasonable leaf drop or other signs of poor health, you may be observing what’s known as decline. The cause of tree decline may not be one specific insect or disease, but a set of cumulative factors.

Forest or tree decline typically means trees are dying in a scattered fashion across a large area over several years. Forest declines are difficult to diagnose and are not possible to manage once they have started. They generally fizzle out on their own, with weaker trees dying and more resilient ones surviving. Declines can be prevented, especially at smaller scales like woodlots and community forests, through management beginning years before potential decline.

Oaks in Minnesota are susceptible to decline if they’re old, growing too close to other trees (thus competing for water and nutrients), shaded, or growing on unfavorable soil (e.g. compacted, very sandy, shallow, or with a high pH). Consecutive years of intense defoliation can also start decline, but the most common inciting factor is severe drought, often spanning several years.

Severe droughts from 2021–2023 have caused widespread decline of oak forests and older yard oaks in a swath from east-central to northwest Minnesota.

Once susceptible forests and trees have been extremely stressed by something like severe drought, they are much more vulnerable to attack by pathogens and insects, which on their own wouldn’t cause serious disease and infestation. Two very common native pests that attack stressed oaks are twolined chestnut borer (Agrilus bilineatus) and Armillaria root disease.

Twolined chestnut borer is a beetle whose larvae live in and feed on the inner bark and cambium (the layer of cells between the bark and the wood). When trees are not stressed, twolined chestnut borer confines its attack to unhealthy trees and broken or shaded branches.

Armillaria root disease is caused by several fungal species of Armillaria. When trees are not stressed, Armillaria survives as rhizomorphs (tough, black, string-like structures) in the soil and on dead tree trunks and roots.

In a true decline phenomenon, the combined damage to an already-stressed tree from (1) multiple years of drought or severe defoliation, (2) twolined chestnut borer, and (3) Armillaria root disease kills the tree, often within two to three years.

Back to top