Look Out for Root Rot in the Landscape

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All the rain for the last few years has caused numerous problems for landscape trees and shrubs. Some plants have completely died, others have had only half the plant die, and still, other plants are just unhealthy and suffering. The main cause of all these problems is drowned roots from saturated soils. 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.

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. However, it can be an aggressive pathogen under some conditions. Severe infections of young plants in nurseries can result in a quick death. Older or mature plants can withstand infections for several years, resulting in a slow decline, eventually ending with the death of the plant.

Above-ground symptoms include leaf drop, dieback, and an overall decline in plant vigor. On conifers, the crowns of infected plants start to thin and change colors, often turning red, brown, or yellowish. Conifer infections sometimes result in heavy resin flow at the tree base.

Armillaria infections start in young roots, but soon the fungus begins to decay larger woody roots. In the early stages of infection, wood becomes slightly grey and can have a water-soaked appearance. Later, the wood becomes light yellow to white and has a soft, spongy rot. Often, rotted areas are offset by black lines of zonation.

Severely diseased trees can pose significant safety hazards during storms since branches and bases are weakened and can easily break under windy conditions or if under added pressure from snow or ice.

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 mushrooms produce microscopic basidiospores, but the spores are not thought to play an important role in the spread of the disease.

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.

Written By

Brad Hardison, N.C. Cooperative ExtensionBrad HardisonExtension Agent, Agriculture - Horticulture, Interim County Extension Director Call Brad Email Brad N.C. Cooperative Extension, Sampson County Center
Updated on Oct 28, 2020
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