Armillaria Root Rot in the Landscape: Attack of the "Humongous Fungus"

Root rot is one of the most commonly diagnosed disease problems of woody plants in landscapes in North Carolina. Each year we see dozens of shrubs and trees that have one of the “big three” root rotting diseases: Phytophthora root rot, Thielaviopsis black root rot, or Armillaria root rot.

This article will feature Armillaria root rot. Armillaria root rot is caused by species of the fungus Armillaria. Common names for this group of fungi include the oak fungi, shoestring root rot fungi, or the honey fungi, the latter referring to the honey-colored mushrooms the fungus produces.

Armillaria is a common soil inhabitant and can infect a very wide host range including; oaks, maples, azaleas, beeches, birches, boxwoods, cedars, dogwoods, firs, poplars, rhododendrons, yews, roses, spruces, and sycamores (pretty much any woody tree or shrub). It can be destructive in orchards or on fruit trees in the landscape.

Armillaria is typically a problem in older plants or plants that have been stressed due to drought, frost, insect attack, mechanical injuries, poor drainage, low soil fertility excessive shade, or pollution damage.

Above-ground symptoms include leaf drop, dieback, and an overall decline in plant vigor. Armillaria infections start in young roots, but soon the fungus begins to decay larger woody roots.

Unlike most plant pathogenic fungi, Armillaria produces mushrooms and other structures that are visible to the naked eye. Three diagnostic signs of Armillaria root rot include:

1. Mycelial Fans: The most common diagnostic sign of this disease can be found beneath the bark (between the bark and the wood) at the base of the tree or shrub. White or creamy paper-like mycelial fans can be observed when the outer bark is carefully peeled away. These white mycelial fans can also be found beneath the bark of infected roots and root collar area.

2. Black Rhizomorphs: Sometimes rhizomorphs (dense strings of mycelium) that look like black shoestrings can be found under the bark or throughout the soil around infected tissue. Rhizomorphs serve as one of the primary means of dissemination. Rhizomorphs grow through the soil from infected trees, roots, or old stumps. They are able to directly penetrate healthy roots and cause disease.

3. Honey Mushrooms: In the fall, honey-colored mushrooms can be seen growing near the base of diseased trees and shrubs. Typically, these mushrooms grow in clusters. These mushroom produce microscopic basidiospores, but the spores are not thought to play an important role in the spread of the disease. Most species of Armillaria are edible and are quite tasty. Caution: It is always best to have any mushrooms identified by an expert before you eat them. Eating misidentified mushrooms can be fatal!

Control

Usually homeowners do not notice Armillaria root rot until the plant is dead or dying. No control is possible at this point and the plant should be removed.

Replanting can be problematic because Armillaria can survive for many years as rhizomorphs in soil or in old wood and stumps. Remove the affected plant and thoroughly dig up and remove all large roots, stumps and any other wood or prunings from the affected area. When planting in areas where a plant has died, or where trees have been removed, as in new construction, remove all old roots, stumps, and wood before replanting. Consider planting ornamental herbaceous or perennial plants or grasses in the area for a few years before attempting to replant woody species.

Healthy trees and shrubs are better able to resist Armillaria root rot than stressed plants. Choose species that are well-adapted to your region and growing site. Maintain their health by fertilizing as recommended, watering during dry spells, and improving drainage in wet areas. When possible, prevent defoliation from insects and foliar diseases. Be careful to avoid damage to roots when digging or tilling. Do not push up soil around tree trunks and do not move soil from affected areas into sites where woody species are growing.

From: Emma Lookabaugh, Plant Disease and Insect Clinic, NCSU

Note: Special thanks to Dr. Larry Grand for helping with this article!