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Armillaria Root Rot of Trees and Shrubs

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Armillaria Root Rot of Trees and Shrubs ALABAMA A&M AND AUBURN UNIVERSITIES Armillaria Root Rot ANR-907 of Trees and Shrubs ushroom root rot, which is Mcaused by fungi in the genus Armillaria is one of the principal causes of tree or shrub death world- wide in forests, commercial plant- ings, nurseries, and landscapes. At the present time, nine species of Armillaria are recognized. In North America, Armillaria mellea or A. tabescens—noted agents of root rot or butt rot—can cause death of seedlings, saplings, Oak leaf hydrangea collapse from and more mature trees or shrubs. Armillaria root rot. These fungi may also act as sec- ondary stress-related pathogens to 8 years. Infection of root systems or cause decay of roots and lower of large trees may progress slowly stems of dead woody plants. In over a period of 10 to 50 years. forested areas, Armillaria root rot is Other root fungi or environmental usually reported on conifers. stresses may cause foliage symp- In the southeastern United toms similar to Armillaria root rot. Butt infections often become Crapemyrtle dieback from Armillaria States, Armillaria root rot has been root rot. reported on many different trees apparent when gum or resin oozes and shrubs such as peach; Japanese from cankers or cracks at or just rounding soil, typically occur at the and red tip photinia; sand pine; ar- above the root collar. Distinct edge of mycelial fans. Flattened rhi- borvitae; camellia; dogwood; elm; semicircular, sunken lesions with zomorphs may also form under the grape; loquat; southern magnolia; cracked margins may be seen bark of the roots or root collar. silver maple; Russian olive; laurel, at or just above the soil line of Mushrooms may appear in fall post, red, turkey, water, and willow trees and shrubs with a thin bark. or early winter on or near the roots oaks; loblolly pine; and pittospo- Heavy resin or gum flows often of diseased plants in clusters of sev- rum. In Alabama, Armillaria-incited develop from root collar cankers eral to 100. Dry fall weather often disease is mainly found in areas on some pines and stone fruit delays mushroom production until where oaks and other hardwoods trees, respectively. Trees damaged the following year. The common are or once were present. by Armillaria may eventually be name, honey fungus, refers to the blown down by wind. honey-colored cap of Armillaria White, often fan-shaped fungal mushrooms. Mushrooms may occur Symptoms mats of growth (mycelium) under Reduced shoot growth, leaf the bark, black threadlike fungal discoloration or dwarfing, stress- rhizomorphs, and characteristic induced production of numerous mushroom clusters are diagnostic small cones or fruit of poor quality, for Armillaria root rot. Fungal (my- and limb dieback are symptomsARCHIVE celial) mats are often thin but may often associated with Armillaria be up to 10 mm thick and are usu- root rot. Severity of foliage symp- ally found at or just above the soil toms depends on the size and age line. Sometimes they may extend of the host as well as the extent of several feet above or below ground root invasion. Usually small trees on trunks or roots, respectively. and shrubs quickly succumb to this Branched rhizomorphs, which are disease while large trees gradually sometimes found on surfaces of Photinia lower trunk section showing decline and die over a period of 2 infected roots, root collars, and sur- white fungal mat of Armillaria fungus just beneath the bark. www.aces.edu around host trees and shrubs lack- in areas formerly occupied by for- ing typical symptoms of this dis- est or orchards or in plants inter- ease. In the absence of characteris- spersed among infected trees. tic or distinctive fungal structures, culture isolations may be used for diagnosis of this disease. Control Armillaria fungi cause a white Good tree and shrub vigor is rot decay of woody tissues. The the best defense against mushroom wood of conifers is initially stained root rot. in landscapes, recom- gray brown and appears water- mended fertilization and watering soaked. As decay progresses, wood Black threadlike rhizomorphs of practices should be followed to becomes yellow brown and stringy. Armillaria on photinia collar and roots. promote plant health and minimize A very wet, stringy texture develops the effects of stress on plants. Trees with pale yellow flecking of the rot- and shrubs should be planted on ted wood. With hardwoods, wood sites where they are best adapted becomes water-soaked, white yel- for growth. Establishing plants off low, spongy, and soft. Although site eventually causes stress-related white rot decay is characteristic of vigor loss which then increases Armillaria, other fungi also cause susceptibility to attack by Armillaria this type of wood rot. fungi. Fungicides will not control this disease. Uprooting stumps and root Disease Development remnants is recommended as a and Spread Armillaria species mushrooms growing means of slowing root rot spread Armillaria fungi are common on and adjacent to a declining oak tree. to nearby healthy trees and shrubs. soil inhabitants and often live in Mushroom root rot spread may be association with their hosts with which link colonized stumps to slowed on diseased trees by remov- no apparent damage. These fungi live trees. Many new infections ing dead bark from the canker and can colonize the cambial tissues of are caused by rhizomorphs which exposing the root collar to air for roots of vigorous healthy shrubs branch off from infected roots and several months. This practice is ef- and trees but are more likely to at- grow into nearby roots of healthy fective only) if the disease is caught tack plants previously weakened by trees. early before the roots are seriously damaged. drought, insect- or disease-caused The rate of disease spread defoliation, mechanical (construc- depends upon the Armillaria If the site is known or suspect- tion) injury to the trunk or roots, or species involved, the amount of ed to have a history of mushroom any other factor that reduces plant inoculum (infected tree roots or root rot, establishing Armillaria- vigor. if the host vigor improves, rhizomorphs), the distance be- resistant plants is recommended. the fungus is often walled off or tween the inoculum and host, and Ginkgo and tulip trees are reported confined to a canker on the roots the environment. Disease spreads to be among the more mushroom or root collar. most quickly in closely spaced root rot-resistant woody plants. Infected tree roots are the plants. Wounds are believed to be Other plants listed as resistant to major source of inoculum, but in- important infection sites for weakly Armillaria root rot include ash, bald fected stumps are also survival sites pathogenic species while viru- cypress, boxwood, catalpa, cherry, and a source of Armillaria. Some lent pathogens can penetrate into Chinese elm, Chinese pistache, Armillaria species only grow in healthy tissues directly. Armillaria crabapple, cryptomeria, dawn close proximity to the roots while fungi will initially infect roots, then redwood, eucalyptus, hackberry, others grow freely through soil, spread into the heartwood. From holly, incense cedar, Leyland cy- forming networks of rhizomorphs there, infection may extend out- press maidenhair tree, maple, pine, ward into inner sapwood. Disease privet, smoke tree, sweetgum, tree- ARCHIVEis often seen in plants established of-heaven white fir, and wisteria. Jacqueline Mullen, Extension Plant Pathologist and Diagnostician, and Austin Hagan, Extension Plant Pathologist, Professor, Entomology and Plant Pathology, both with Auburn University For more information, call your county Extension office. Look in your telephone directory under your county’s name to find the number. Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, reli- gion, sex, age, veteran status, or disability. 3.5M, Revised Oct 2004, ANR-907 ANR-907 © 2004 by the Alabama Cooperative Extension System. All rights reserved..
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