How to Manage Pests
UC Pest Management Guidelines
Armillaria Root Rot (Oak Root Fungus)
Pathogen: Armillaria mellea
In this Guideline:
Roots infected with Armillaria mellea have white to yellowish fan-shaped mycelial mats between the bark and the wood. Dark brown to
black rhizomorphs sometimes can be seen on the root surface. All
stone fruit rootstocks are susceptible to Armillaria root rot. The plum
rootstock Marianna 2624 has some tolerance and may be useful in some
The fungus survives on dead roots.
Avoid planting peach orchards where forest or oak
woodland has recently been cleared or where there is a history of Armillaria
root rot. All rootstocks can be attacked by Armillaria mellea but some, such as Marianna 2624, are less
affected than others. Maintain the vigor of the trees to help resist Armillaria attack. Infested sites can be fumigated, but
often this procedure will not prevent recurrence of the disease.Physical barriers to contain infection centers
have been used successfully in orchards. Four-foot trenches are dug around the
infected trees and plastic tarp is laid inside the trench wall from bottom to
top before the soil is replaced. The tarp prevents healthy roots from coming in
contact with diseased ones, thus preventing spread of the disease.
Exposing an infected crown and upper root area of
a tree infected with Armillaria mellea may stop the development of the fungus into the
crown area and allow the tree to regrow. Remove soil from around the base of
the tree to a depth of 9 to 12 inches. Leave the trunk exposed and keep the
upper roots and crown area as dry as possible. During winter, provide drainage
so that rain does not collect in the hole. Recheck the hole every few years to
make sure it has not filled with leaves, soil, and other matter; the hole must
be kept open and the crown and upper roots exposed. Some rootstocks may produce
suckers when exposed, and these will need to be removed.
Before fumigation, remove all infected trees,
stumps, and as many roots greater than 1 inch in diameter as possible.
Healthy-appearing trees adjacent to those showing symptoms are often infected
also. Removal of these adjacent trees and inclusion of that ground in the soil
fumigation may be advisable. Infected trees, stumps, and roots should be burned
at the site or disposed of in areas where flood waters cannot wash them to
agricultural lands. Complete eradication is rarely achieved, and retreatment
may be necessary in localized areas. If the soil is wet or if it has extensive
clay layers to the depths reached by the roots, fumigant treatment may not be
successful. The greatest opportunity for eradication occurs on shallow soils
less than 5 feet in depth. Fumigate from late summer to early fall.
||Amount to Use
choosing a pesticide, consider information relating to environmental impact Not all registered pesticides are listed. Always read label of product being used.
For preplant fumigation. Must be applied under a Critical Use Exemption only.
Before fumigating, dry soil by withholding water during summer and using
cover crops such as sudangrass or safflower. The drier the soil the better
for deep penetration. Deep-till the area after drying. If the soil is dusty,
wait for an early rain before ripping and fumigation. Ripping a dry soil that
is silty can result in large clods on the surface. Inject methyl bromide
18–30 inches deep with chisels and cover with gas-proof cover.
Increasing the dose tends to increase the depth of penetration, but it cannot
be relied upon to penetrate wet soils, especially if soils are high in clay.
Do not remove the cover for at least 2 weeks and aerate 1 month before
planting. Fumigants such as methyl bromide are a prime source of volatile
organic compounds (VOCs), which are a major air quality issue. Fumigate only
as a last resort when other management strategies have not been successful or are not available.
ACTION GROUP NAME (NUMBER1): Unknown. A thiocarbonate fungicide.
Trees must be in the ground at least 1 year before treatment or injury may occur. See label for treatment timing.
UC IPM Pest Management Guidelines: Prune
UC ANR Publication 3464
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside
C. Pickel, UC IPM Program, Sutter/Yuba counties
W. H. Olson, UC Cooperative Extension, Butte County
F. J. A. Niederholzer, UC Cooperative Extension, Sutter/Yuba counties
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier
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