UC IPM: UC Management Guidelines for Armillaria Root Rot on Nectarine

UC Pest Management Guidelines

Nectarine

Armillaria Root Rot (Oak Root Fungus)

Pathogen: Armillaria mellea

(Reviewed 6/06, updated 6/06)

In this Guideline:

Symptoms
Comments on the disease
Management

SYMPTOMS

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 sometimes to Armillaria root rot. The plum rootstock Marianna 2624 has some tolerance and may be useful in some situations.

COMMENTS ON THE DISEASE

The fungus survives on dead roots.

Avoid planting nectarine 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 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 infection center 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.

Cultural Control
Research on 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. In spring, remove soil from around the base of the tree to a depth of 9 to 12 inches. Leave the trunk exposed for the remainder of the growing season. During the spring, summer, and fall, keep the upper roots and crown area as dry as possible. During winter, provide drainage if necessary 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.

Organically Acceptable Methods
Cultural controls are acceptable for use in an organically certified crop.

Chemical Control
Before fumigating, 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.

Common name

(trade name)

Amount to Use

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

METHYL BROMIDE*

Label rates

COMMENTS: Must be applied under a Critical Use Exemption. For preplant fumigation. 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.

SODIUM TETRATHIOCARBONATE

(Enzone)

Label rates

MODE OF ACTION: Unknown. A thiocarbonate fungicide.

COMMENTS: Trees must be in the ground at least 1 year before treatment or injury may occur. See label for treatment timing.

*	Permit required from county agricultural commissioner for purchase or use.
¹	Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different Group number are suitable to alternate in a resistance management program. For more information, see http://www.frac.info/.

UC IPM Pest Management Guidelines: Nectarine
UC ANR Publication 3451
Diseases
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to the diseases section:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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