Pacific Northwest 1998 An Online Guide to Plant Disease Control

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Blackberry -- Armillaria Root Rot
See Also: Oak -- Armillaria Root Rot
 
Cause: Armillaria mellea, a fungus, infects blackberry roots, killing the cambium and decaying the underlying xylem. Often found on newly cleared land, this root pathogen is native to the PNW where it occurs on the roots of many forest tree species including Douglas-fir, madrone, oak, willow, and yellow pine. It also attacks black and red raspberries and trailing berries. Other agronomic hosts include currants, gooseberries, grapes, nut trees, roses, strawberries, stone fruits and many rosaceous plants. The host range includes over 500 species of woody plants, making its common name of "oak root fungus" slightly misleading.

This fungus may form mushrooms at the base of infected bushes in fall and winter. Mushrooms produce wind-blown spores, but these spores are not a significant means of infecting healthy blackberries. The fungus spreads vegetatively, below ground, which leads to the formation of groups of dead and dying plants called “disease centers.” The fungus can survive on woody host roots long after the host dies. Its vegetative fungal tissue (mycelium) decomposes root wood for nutrients as it grows. When infected plants are removed, infected roots that remain below ground serve as a source of inoculum for blackberries planted in the same location.

Infection occurs when blackberry roots come in direct contact with partially decayed tree roots and are colonized by mycelium. Infection can also occur when blackberry roots contact rhizomorphs (black, shoestring-like fungal structures) that grow out from partially decayed roots and through the soil. Once blackberry roots and crowns are infected, whether they are living or dead, they serve as a source of inoculum for neighboring blackberries. Fresh sawdust mulch may also contain the pathogen.

Symptoms: The first visible disease symptom is a decline and dieback in which leaves turn yellow, wilt, and die. This may occur only on one side of the plant or in one or two canes. Diagnostic mycelial fans can be observed beneath the bark of the root crown of infected plants. Dig down about a foot below the soil line and use a pocketknife to remove thin layers of bark from the root collar. Mycelial fans are thick, white layers of fungus that adhere to the root bark and/or the wood beneath the bark. Armillaria also makes black, shoestring-like structures called rhizomorphs, which are occasionally found within the bark and/or extending into surrounding soil. Rhizomorphs may look like roots on the outside but have an entirely different structure when cut open in cross-section.
Cultural control:

  1. When clearing a new site of forest trees and shrubs or when clearing a site with infected plants (infection centers) there are several precautions to take.
    1. Girdle large trees before removal to hasten decay of roots.
    2. After removing aboveground vegetation, clear soil of stumps and large roots. Deep-rip the soil in more than one direction to bring large roots to the soil surface. If possible, remove all roots greater than 1 inch in diameter from the soil.
    3. Burn all woody debris.
    4. Leave this ground fallow at least 1 year. Trenches lined with plastic sheeting may help if inoculum is from an adjacent stand.
    5. If using drip irrigation, move drip-line emitters away from the crown and in between blackberries after first year of planting.
    6. Remove severely infected bushes and destroy them, being careful to remove small roots from the soil. If practical, do not replant where infected bushes have been removed.
Chemical control: Soil fumigation is most effective if preceded by thoroughly clearing the soil of woody debris. Methyl bromide fumigation has been found to give the best, albeit limited, control. Methyl bromide is more effective if soil is extremely dry. It works better on fine soils with few rocks. Restricted-use pesticide.
  1. Contrary to soil fumigation for nematodes, soil should be as warm and dry as possible. Fumigation in late summer before any rain is best.
  2. Apply fumigant as deeply as possible.
  3. Follow directions for application, sealing, aerating, and waiting period before planting.
  4. Spot fumigation may be necessary a few years after planting.
References:
Baumgartner, K. and D.M. Rizzo. 2001. Distribution of Armillaria species in California. Mycologia 93:821-830.

Hood, I.A., D.B. Redfern, and G.A. Kile. 1991. Armillaria in Planted Hosts. In: Shaw, C.G. III and G.A. Kile, eds. Armillaria Root Disease. Washington, D.C.: United States Department of Agriculture Forest Service Agriculture Handbook No. 961. p 122-149.

Munnecke, D.E., M.J. Kolbezen, W.D. Wilbur, and H.D. Ohr. 1981. Interactions involved in controlling Armillaria mellea. Plant Disease 65:384-389.

Content edited by: Jay W. Pscheidt on January 1, 2009
 
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