Root
and Butt Rot of Oaks
L. F. Grand, Research Plant Pathologist
R. K. Jones, Extension Plant Pathologist
[Cause] [Hosts] [Symptoms] [Signs] [Control] [Other Links]
Within the past decade there has been an increase in root and butt rot
of old oak trees in North Carolina, especially in urban landscapes, parks
and along streets. Although several species of oak have been involved,
willow oak and water oak are the most frequently affected. These two oak
species were commonly planted along streets and in the landscape in the
early to mid 1900s, and normally live 65-80 years. These trees are succumbing
to a variety of problems as they reach their life-expectancy.
Several species of wood decay fungi cause root and butt rot of oaks. In the southeastern United States, the following fungi have been reported on different oak species: Armillaria spp., Bondazewia berkeleyi (Polyporus berkeleyi), Ganoderma lucidum (curtisii), Grifola frondosa (Polyporus frondosa), Inonotus dryadeus (Polyporus dryadeus), I. dryophilis, Laetiporus sulphureus(Polyporus sulphureus), and Meripilus sumstinei (Polyporus giganteus). Inonotus dryadeus is one of the most common of these wood decay species.
Most oak species are susceptible to root and butt rot by the various species of fungi listed above. The following oak species have been reported as hosts for the most commonly encountered fungus, Inonotus dryadeus: Quercus alba (white), Q. coccinea (scarlet), Q. nigra (water), Q. phellos (willow), Q. prinus (chestnut), Q. rubra (red), Q. shumardii (Shumard), Q. stellata (post), and Q. velutina (black). Older long-lived oak species (white and red oaks) and short-lived species (willow and water oaks) are most frequently affected.
Blow-down
during rainstorms or windy periods is often the first and only indication
of root rot. Sparse foliage with limb dieback also may be symptomatic
of root rot but are not consistently associated with the disease prior
to blow-down. Advanced decay of the larger roots, especially the tap or
anchor roots, is evident after blow-down. Decay may extend from a few
inches to several feet into the butt portion of the tree, depending on
the species of fungus involved. Decay may be of the white rot type, characterized
by whitish to straw-colored, wet, stringy wood; or of the brown rot type,
characterized by brown, dry, crumbly wood often with horizontal and vertical
fissures. See Table 1 for the types of rots
produced by the various fungi associated with root and butt rot of oaks.
One of the most important indicators of the presence of root rot prior to blow-down is the presence of basidiocarps (reproductive bodies) of the fungi. These may be of two general types. One type, usually referred to as conks, or shelf or bracket fungi, is more or less woody in texture, shelf-like, with or without a stem, and with tiny pores on the underside. The basidiocarps of the second type are mushrooms, with stems and caps with gills on the underside. The basidiocarps are usually formed at or near the base of the tree, but also may be formed 3 to 6 feet or more from the base, coming from decaying roots. General descriptions of the more common species of fungi associated with root and butt rot of oaks are given in Table 1.
Studies have indicated that severity of decay can be estimated by: 1. Presence of basidiocarps; 2. Number of basidiocarps-the greater the number, the more decay; 3. Size of basidiocarps-the larger the basidiocarp of a given species, the more decay; and 4. Distribution of basidiocarps around the tree-the larger the percentage of the circumference found, the more decay.
In addition to the presence of basidiocarps, root rot caused by Armillaria spp. can be identified by the presence of white, fan-shaped mats of fungal tissue (mycelium) beneath the bark at the base of the tree or on larger roots, and by black, shoestring-like structures (rhizomorphs) that can be found beneath and on the surface of the bark and in the soil.
Prevention is difficult due to the longevity of the oaks and the locations where the disease is frequently found. The fungus enters the tree through wounds so any precaution that would reduce injuries to the roots or base of the tree is advisable. However, prevention of injuries over several decades in landscape situations is difficult or even impossible.
Prompt action
upon diagnosis of the disease is paramount. Since positive diagnosis is
dependent on presence of the basidiocarps, decay is usually well advanced
at this time. Removal of the affected tree to avoid damage to surrounding
property is recommended as soon as possible. Blown-down trees can cause
considerable damage to property.
Table 1. Description of basidiocarps and type of decay produced by the more common species of fungi causing root and butt rot of oaks.
|
Fungus
|
Type
of rot
|
Description
|
|
|
Armillaria
spp.
|
White, yellow brown, stringy | Brown mushroom, in clusters, with a ring on stem |
 |
|
Bondarzewia
berkeleyi
|
White, stringy | Large, multibranched, cream top |  |
| White | Orange-maroon, "varnished" top, hard, with or without stem |  |
|
| White | Large, multibranched, gray top, fibrous |  |
|
| White | Very large, cream top becoming brown |  |
|
| White | Brown to reddish brown, 3-6 feet up the trunk |  |
|
|
Laetiporus
sulphureus
|
Brown cubical | Large, bright yellow to yellow orange |
 |
| White | Large, multibranched, gray to brown top |  |
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