UC IPM: UC Management Guidelines for Armillaria Root Rot (Oak Root Fungus) on Almond

UC Pest Management Guidelines

White mycelial mats are evident at or below ground in trees infected with Armillaria.

Almond

Armillaria Root Rot (Oak Root Fungus)

Pathogen: Armillaria mellea

(Reviewed 3/09, updated 3/09)

In this Guideline:

Symptoms
Comments on the disease
Management

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. Infected trees develop pale foliage with small leaves, a lack of new growth, and a thin canopy, usually followed by sudden death when the first hot weather of early summer arrives.

The fungus survives on dead roots. It spreads from one tree to another through close contact of diseased roots with healthy roots. All stone fruit rootstocks are susceptible to Armillaria root rot. The plum rootstock Marianna 2624 is the most resistant to the fungus, but it is not immune. Use of this rootstock is the only practical alternative if almonds are to be grown in soils where Armillaria has infected roots and killed trees on other rootstocks. Wet soil conditions resulting from heavy rainfall or excessive irrigations can exacerbate the disease.

The only treatment is fumigation. Before chemical treatment, 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. Treat Armillaria from late summer to early fall.

Common name

Amount/Acre

R.E.I.+

P.H.I.+

(trade name)

(hours)

(days)

When choosing a pesticide, consider information relating to environmental impact.

PREPLANT

METHYL BROMIDE*

Label rates

see label

COMMENTS: Any use of methyl bromide must be allowed under a current Critical Use Exemption. 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. After drying, deep-till just the area that will be fumigated. 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 to 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.

SODIUM TETRATHIOCARBONATE

(Enzone)

Label rates

4 days

MODE OF ACTION GROUP NAME (NUMBER¹): Unknown. A thiocarbonate fungicide.

COMMENTS: Make two applications to moist soil. Be sure to remove large roots as well as smaller roots (pencil size) from soil planting site (10 x 10 ft area) before treatment.

Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.

Permit required from county agricultural commissioner for purchase or use.

Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431
Diseases
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside
Roger Duncan, UC Cooperative Extension, Stanislaus County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center
B. A. Holtz, UC Cooperative Extension, Madera County
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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How to Manage Pests

UC Pest Management Guidelines

Almond

Armillaria Root Rot (Oak Root Fungus)

Pathogen: Armillaria mellea

SYMPTOMS

COMMENTS ON THE DISEASE

MANAGEMENT

PUBLICATION