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.
The fungus survives on dead roots.
Avoid planting plum orchards where forest or oak woodland has
recently been cleared or where there is a history of Armillaria root rot. All
stone fruit rootstocks are susceptible to Armillaria root rot but some, such as
Marianna 2624, are less affected than others and may be useful in some
situations. 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. A 4-foot
trench is 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.
other tree crops has indicated that exposing an infected crown and upper root
area of a tree infected with Armillaria mellea may help to slow
the development of the fungus into the crown area. Remove soil from around the
base of the tree to a depth of 9-12 inches. Leave the trunk exposed and keep
the upper roots and crown area as dry as possible. During winter, provide
drainage so that rain doesn't collect in the hole. Recheck the hole every few
years to make sure it has not filled in 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.
Organically Acceptable Methods
controls are acceptable in organically managed orchards.
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
|When choosing a pesticide, consider
information relating to environmental quality.
||COMMENTS: For preplant
fumigation; must be applied under a Critical Use Exemption. 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 source of volatile organic compounds (VOCs) but are
not reactive with other air contaminants that form ozone; methyl bromide
depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
||MODE OF ACTION: Unknown. A thiocarbonate insecticide.
||COMMENTS: Trees must be in the ground at
least in the ground 1 year before treatment or injury may occur. See label for treatment timing.
UC IPM Pest Management Guidelines: Plum
UC ANR Publication 3462
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside
K. R. Day, UC Cooperative Extension, Tulare County
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier
Top of page