ARMILLARIA (SHOESTRING) ROOT ROT
Shoestring root rot is caused by a group of fungi known as Armillaria. At least 12 species of Armillaria have been shown to cause root rots, but since it is very difficult to distinguish between these fungi, and many of these disease situations have not been thoroughly investigated, they are commonly refered to only as Armillaria melea.
These fungi rot the roots of many different kinds of plants. Most often this disease is found on trees and shrubs such as fir, oak, pine, rhododendron, lilac, and dogwood. However, it is not restricted to woody plants and has been found on raspberry and strawberry. Plants which are not growing well are more likely to be seriously damaged.
Symptoms of this root rot on above-ground parts of a plant generally appear as stunting, yellowing, or browning of leaves of needles, which may drop. Symptoms occur over the whole plant. Foliage may look unhealthy and become more sparse over a period of several years or may show no evidence of any problems but suddenly die. Similar symptoms may be caused by other factors such as general lack of plant care or weather stress.
Armillaria root rot can be distinguished from other problems by examining the lower trunk and roots. If Armillaria is responsible for the problem, a white, generally felt-like fungus growth can be seen between the bark and the wood when the bark is carefully peeled from the wood. At the edge of a diseased area, the white fungus growth normally assumes a characteristic fan-shape. The fungus growth may also be in bark (Figs. 1 and 2).
In addition, the fungus forms blackish, string-like strands about 1/16 inch in diameter or less which can often be seen between the bark and the wood, and/or on the surface of the roots, and/or in the adjacent soil. These string-like strands, called shoestrings, look similar to roots and may be confused with the roots of the plant, or adjacent plants. However, the shoestrings are darker and grow from areas and in ways which roots would not (Figs. 1 and 3). Honey-colored mushrooms may develop around the base of the affected plant, but many times these do not develop.
Fig. 1. Lilac stump and root with Armillaria root rot. The root (on left) has had bark removed, revealing the white fan-shaped fungus growth. In front of the stump, a small pile of shoestrings of the fungus. Some shoestrings are still attached to the stump.
Severely affected and dying plants.
There is no cure for severely affected plants. They should be removed and destroyed as soon as possible. Control is directed toward stopping further spread of the disease, or protecting the replacement planting from infection.
It is important to remove and destroy (by burning, if possible) all of the stump and root system, even small roots, because the fungus can live on and in the infected stump and roots for many years. Using this wood as a food source, the shoestrings of the fungus (Figs. 1 and 3) grow through the soil and can infect adjacent plants. Healthy appearing plants growing near obviously diseased plants may already be infected. Armillaria is common when forest lands have been recently cleared and infected stumps and roots are buried. New plantings in the vicinity of infected debris are likely to become diseased. Shovels, axes, and other tools used to cut up and dig up the plant should be cleaned and/or sterilized after use to avoid spreading the fungus.
In certain situations, soil fumigation can help in controlling the spread of the disease, especially in lighter soils. Fumigation should be done by a licensed pesticide applicator. To be effective, the roots must be removed prior to treatment. Roots of adjacent plants can be injured by this treatment, so fumigation is not advised when adjacent healthy plant roots are in the area to be treated. If fumigation is not used, it may be beneficial in limited situations to replace the contaminated (fungus-infested) soil with fresh soil. Avoid moving contaminated soil to uncontaminated areas, since this may spread the fungus. Clean contaminated tools as mentioned above.
Slightly affected plants.
Trees and shrubs which are not seriously affected may be helped. The soil should be removed from around the rotted parts of the trunk and larger roots to allow them to air dry. The infected areas should be cut out down to the healthy tissue and diseased tissues destroyed. Wash cutting tools in soap and water and sterilize them in rubbing alcohol afterward. Roots should be left exposed during summer, but covered over before freezing fall weather. The plant should be given proper fertilization and watering to promote good growth; however, avoid watering the exposed trunk and larger roots to keep this area of the plant as dry as possible. Whether or not the trunk and larger roots have been exposed for drying, always avoid watering trees and large shrubs where the trunk enters the ground.
If a plant is moderately affected and of special value to the owner, follow the above procedure. However, the greater the area of infection, the less chance the plant has of surviving.
New plantings for affected areas.
Because Armillaria infects about 700 different plant species, planting new plants in areas where infected ones have been removed may be risky. If such planting is done, the possibility of infection of the new plants should be considered. The more thorough the removal of the infected plant (including its root system) and associated fungus structures, the better chances will be for the health of the second plant. Be sure to give the new plant good cultural care.
Information regarding plants resistant (meaning they don't often get the disease) to this disease can sometimes be misleading since there are several different species of Armillaria that vary in the plants they can attack. In addition, the soil type and the environment can influence how a plant will react to disease pressure. Keeping that in mind, the following plants have been reported to be resistant to Armillaria root rot. However, this information should only be used as a general guideline.
Many common herbaceous plants (annuals, bulbs, etc.) can be rotted by Armillaria, but lawn grasses common to our area are not reported to be affected by it. If a resistant woody plant cannot be found, extending the lawn to cover the affected area would seem to be a suitable option.
Fig. 2. Lower trunk and upper root of a small
fir with Armillaria root rot. Bark has been cut to
reveal white, fan-shaped fungus growth within the bark and between bark and wood.
Fig. 3. Shoestrings of the fungus which
causes Armillaria (shoestring) root rot.
These fungus structures grow through
the soil to infect nearby plants.
Some plants that are resistant to Armillaria root rot:
TREES AND SHRUBS
Common name Abies concolor White fir Jacaranda acutifolia Jacaranda Acacia longifolia Bush acacia Liquidambar orientalis Oriental sweet gum Acacia mearnsii Black wattle Liquidambar styraciflua Sweet gum Acacia verticillata Star acacia Liriodendron tulipifera Tuliptree Acer macrophyllum Bigleaf maple Lonicera nitida Box honeysuckle Acer palmatum Japanese maple Magnolia grandiflora Southern magnolia Ailanthus altissima Tree-of-heaven Mahonia aquifolium Oregon grape Arbutus menziesii Madrone Malus floribunda Japanese flowering crab Berberis polyantha Barberry Maytenus boaria Mayten tree Betula pumila Swamp birch Metasequoia glyptostroboides Dawn redwood Buxus sempervirens Boxwood Morus species Mulberry Calocedrus decurrens Incense cedar Myrica pensylvanica Bayberry Catalpa bignonioides Common catalpa Nandina domestica Palms Heavenly bamboo Celtis species Hackberry Pinus canariensis Canary Island pine Ceratonia siliqua Carob Pinus nigra Austrian pine Cercis occidentalis California redbud Pinus radiata Monterey pine Cercis siliquastrum Judas tree Pinus sylvestris Scotch pine Chaenomeles lagenaria Japanese quince Pinus torreyana Torrey pine Chamaecyparis lawsoniana 'Ellwoodii' Elwood cypress Pistacia chinensis Chinese pistache Cotinus coggygria Smoke tree Pittosporum
Queensland pittosporum Cryptomeria japonica Japanese cedar Platanus species Sycamore Cupaniopsis anacardioides Carrotwood Prunus caroliniana Cherry laurel Cupressocyparis leylandii Leyland cypress Prunus ilicifolia Holly-leaved cherry Cupressus arizonica var. glabra Smooth Arizona cypress Prunus lyonii Catalina cherry Elaeagnus angustifolia Russian olive Quercus ilex Holly oak Erica arborea Tree heath Quercus lobata Valley oak Eucalyptus camaldulensis Red gum Raphiolepis umbellata Yedda hawthorn Eucalyptus cinerea Silver-dollar tree Rhus aromatica Fragrant sumac Eugenia species Eugenia Sambucus canadensis American elder Fraxinus uhdei Evergreen ash Sequoia sempervirens Coast redwood Fraxinus velutina var. glabra 'Modesto' Modesto ash Sophora japonica Japanese pagoda tree Ginkgo biloba Ginkgo Taxodium distichum Bald cypress Gleditsia triacanthos 'Shademaster' Shademaster locust Ternstroemia species Ternstroemia Hibiscus syriacus Rose-of-Sharon Ulmus parvifolia Chinese elm Hypericum patulum St. John's wort Vitex agnus-castus Chaste tree Ilex aquifolium English holly Wisteria sinensis Chinese wisteria
FRUITS, NUTS, AND BERRIES
Common name Carya illinoinensis Pecan Malus species Crabapple Castanea dentata American chestnut Persea americana Avocado Diospyros kaki Japanese persimmon Prunus cerasifera Cherry plum Diospyros virginiana Common persimmon Prunus serotina salicifolia Black cherry Ficus carica 'Kadota' Kadota fig Pyrus calleryana Callery pear Ficus carica 'Mission' Mission fig Pyrus communis Pear Juglans hindsii Black walnut Rubus ursinus loganobaccus Loganberry Malus pumila Common apple Rubus ursinus Ollalie
Prepared by Roy M. Davidson, Jr., former agricultural research technologist, and Ralph S. Byther, Extension plant pathologist (retired), WSU Puyallup.
College of Agriculture and Home Economics, Pullman, Washington
Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Revised from and replaces EM4433. May 1994. Subject Code 356. A. EB1776