How to Manage Pests

UC Pest Management Guidelines

Walnut

Armillaria Root Rot (Oak Root Fungus)

Pathogen: Armillaria mellea

(Reviewed 12/07, updated 4/09)

In this Guideline:


SYMPTOMS AND SIGNS

Trees with Armillaria root rot have thin canopies with yellow leaves and twig, shoot, or limb dieback. Infected roots have white rot wood decay with white to yellowish fan-shaped mycelial mats (plaques) between the bark and the wood. Dark brown to black rhizomorphs sometimes can be seen on the root surface.

COMMENTS ON THE DISEASE

Armillaria survives on dead roots. Paradox, Northern California Black, and English rootstocks are susceptible, but disease response is variable.

MANAGEMENT

Management of Armillaria root and crown rot relies primarily on preventing infection of new trees. You can reduce the chances of infection by carefully preparing planting sites for new orchards and by practicing good sanitation and early detection. Do not rip or disc in infected orchards to avoid spreading the inoculum. Overly wet soil conditions favor development of this disease, so take measures to correct this condition throughout the orchard.

Tolerant rootstocks. Choose a less-susceptible rootstock if Armillaria root rot is present. Avoid using English rootstock, which is highly susceptible, in sites known to be infested with A. mellea. Paradox rootstock is generally more tolerant of Armillaria root rot than Northern California Black rootstock. However, neither of these rootstocks is tolerant if an extremely virulent strain of Armillaria is present.

Fumigation. If possible, avoid a planting site infested with Armillaria. Where this is not possible, fumigate before planting. Be aware that fumigants do not kill all infected roots deep in the soil. See your Cooperative Extension farm advisor for additional advice on soil preparation, type of fumigants, application, and precautions, and follow directions and regulations carefully. Fumigation is expensive and needs to be done correctly for maximum benefit.

Sanitation. Be careful not to introduce Armillaria into an established orchard via diseased root pieces that may be transported on equipment or in surface water.

Monitoring. Check trees for symptoms of Armillaria root rot in late summer when dead or declining trees are most obvious. Look for mycelial plaques under bark or mushrooms at the bases of trees after a rain, generally from October to April.

Saving infected trees. Once symptoms of Armillaria root rot appear, it may be possible to slow or stop spread of the pathogen within an infected tree by exposing the crown and upper roots and allowing them to dry out. Remove soil from the base down to a depth of 9 to 12 inches in spring. Keep the crown and upper roots exposed to the air and avoid wetting them for the duration of the growing season. Fill the soil back in before rains start in the fall. This procedure may allow the diseased tree to regrow, but is not always successful. If trees cannot be saved, remove infected trees and apparently healthy trees in several adjacent rows; once symptoms appear on one tree, the disease is likely to have already spread to the roots of the surrounding trees.
Common name   R.E.I.+ P.H.I.+
(trade name) Amount/Acre (hours) (days)

  Calculate impact of pesticide on air quality
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PREPLANT
A. METHYL BROMIDE* Label rates 48 NA
  COMMENTS: Use allowed under a Critical Use Exemption only. To prepare for fumigation, remove and destroy all roots larger than 1 inch in diameter. 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. 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.
   
+ 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 these two intervals is the minimum time that must elapse before harvest may occur.
* Permit required from county agricultural commissioner for purchase or use.
NA Not applicable.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Diseases

  • J. E. Adaskaveg, Plant
  • Pathology, UC Riverside
  • R. P. Buchner, UC Cooperative Extension, Tehama County
  • G. T. Browne, USDA Crops Pathology and Genetics, UC Davis
  • W. D. Gubler, Plant Pathology, UC Davis
Acknowledgement for contributions to Diseases:
  • B. L. Teviotdale, Kearney Agricultural Center, Parlier

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