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Armillaria root rot

curled root amongst woodchips

White mycellia of Armillaria can be seen under the bark of this root

M. Grabowski, UMN Extension

Armillaria root rot is an uncommon disease that usually occurs in blueberries that were planted in sites where one or more oak trees were removed. Although uncommon, the disease can weaken and kill a blueberry plant.


tree trunk with section of long, curling black lines

Thick black rhizomorphs of Armillaria growing on an old tree trunk

M. Grabowski, UMN Extension

In the early stages, Armillaria root rot resembles a nutrient deficiency or drought stress. Leaves are pale yellow to white with green veins or have brown edges. Plant vigor will be low. Leaves may be small and little shoot growth will be observed. As disease becomes severe, leaves wilt and the entire plant dies.

If the bark is peeled away from the base of the stem or some of the major roots, flat white sheets of fungal growth will be seen between the bark and the wood. Nearby stumps or dead trees may provide further signs of Armillaria. Thick black, shoestring-like fungal strands called rhizomorphs sometimes grow in a net on infected trees and in the soil around the base of the tree. In fall, honey colored mushrooms, with a round cap, white gills and a ring around the upper stem grow on or near infected trees. Although Armillaria mushrooms are uncommon on infected blueberry plants, their presence on nearby trees is a sign that the pathogen is in the area.

Important biology

Pale yellow 'honey' colored mushrooms of Armillaria

M. Grabowski, UMN Extension

Armillaria root rot can be caused by several species of the fungus Armillaria. Armillaria mellea is the most common species found infecting deciduous trees and shrubs. Armillaria spp. can infect over 500 species of trees and shrubs and is commonly associated with oak roots.

Armillaria can survive as a root rotting pathogen on living plants and as a saprophyte on dead wood. Blueberries become infected when their roots come in contact with the infected roots of another tree or shrub, infected plant debris or fungal rhizomorphs. It is not uncommon for infected plants to be right next to healthy plants.


Avoid using wood chips from oak trees for mulch, especially if the oak trees were diseased or rotting. Blueberry plants killed by Armillaria should be removed and burned. If an oak tree is killed, remove as many roots as possible along with the stump before planting blueberries in the area.

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