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

UC Pest Management Guidelines

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

Pistachio

Armillaria Root Rot (Oak Root Fungus)

Pathogen: Armillaria mellea

(Reviewed 2/07, updated 2/07)

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 can sometimes be seen on the root surface. Although pistachio is susceptible to Armillaria root rot, the disease is infrequently reported. The relative susceptibility of pistachio rootstocks is unknown.

COMMENTS ON THE DISEASE

The fungus survives on dead roots.

MANAGEMENT

Armillaria root rot is only occasionally a problem in pistachio orchards. Management of this disease can be difficult and resistant rootstock may offer the best protection. Research indicates that Pistacia terebinthus and P. atlantica x P. integerrima hybrids are tolerant but P. atlantica and P. integerrima are susceptible. However, because pathogenicity and virulence in the natural population of the pathogen ranges from weak to severe, disease response may vary with different combinations of rootstocks and pathogen. Exposing an infected crown may help individual trees with the disease. Soil fumigation as a preplant treatment can slow progress of the disease, but eradication is not likely.

Treatment Decisions
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.+

(trade name)

(hours)

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

PREPLANT

METHYL BROMIDE*

Label rates

COMMENTS: Use permitted under a Critical Use Exemption permit only. 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 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 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.

*	Permit required from county agricultural commissioner for purchase or use.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be entered without protective clothing.

UC IPM Pest Management Guidelines: Pistachio
UC ANR Publication 3461
Diseases
T. J. Michailides, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to the diseases section:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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