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Armillaria Root Rot

Armillaria mellea

Armillaria root rot infects many crops and native and orna-

mental plants. Common hosts include avocado, cherimoya,

citrus, and oak trees. The fungus persists in infested roots and wood in soil, infecting new plantings and spreading to infect

nearby plants (Figure 21). Symptoms and Damage

The Armillaria fungus can become well established in roots

and the root crown before any symptoms are visible above

ground. Infected trees usually die prematurely, and if they

are young trees they often die quickly after infection. Mature

trees may die quickly or slowly, or they may recover at least

temporarily if conditions become good for tree growth and

poor for disease development.

Wilted, downward-hanging foliage is often the first obvi-

ous symptom of Armillaria root rot. Other symptoms include

yellowing of the foliage, leaf drop, and dieback of upper limbs.

During rainy fall and winter periods, short-lived mushrooms

often appear around the base of Armillaria-infected trees. The

mushroom caps vary in color from off-white to honey-yellow

to almost black. Each cap is about 1 to 10 inches (2.5–25 cm)

in diameter. The mushrooms always occur in groups, never

singly. Mushrooms have a ring on the stalk just under the cap

The most reliable sign of Armillaria root rot is white, cottony fungal

mycelial growth in cambial tissue. If trees exhibit aboveground symp-

toms of infection, expose the root crown and cut under the crown's

bark to look for Armillaria mycelium. During the rainy fall and winter, short-lived mushrooms often grow

around the base of Armillaria-infected trees such as this almond. Scout

your groves for these mushrooms after rains. Mark any trees where mush-

rooms occur, confirm whether the cause is Armillaria root rot, and then

develop and implement a management plan.

This wilted, downward-hanging foliage is a symptom of infection by

Armillaria mellea. Other symptoms of Armillaria root rot include

yellowing of foliage, leaf drop, and dieback of upper limbs.

JACK KELLY CLARK

Excerpted from Integrated Pest Management for Avocados, ANR Publication 3503

See www.ucanr.org/ipm-avocado

©2008 by the Regents of the University of California

and shed numerous minuscule, white spores. Spores do not

appear to be an important source of infection in California

avocados.

The most reliable sign of Armillaria root rot is a white

growth of fungal mycelium in the cambial tissue. If trees

exhibit aboveground symptoms of infection, cut under the

bark of the root crown and major roots to check for myce-

lium, which are whitish and have a strong mushroom odor.

Growth typically occurs in patches in the cambium and

inner bark.

Seasonal Development

Armillaria mycelium persists for years under the bark of a

tree’s diseased roots or root crown. The fungus spreads from

tree to tree mainly by means of natural root-to-root grafts

and by cordlike rhizomorphs, which resemble small, dark

roots. In contrast, healthy avocado roots are lighter colored,

usually light brown to whitish. When pulled apart, rhizo-

morphs have a cottony interior, while the center of a healthy

root is solid and woody. Rhizomorphs grow along or out

from diseased roots, eventually contacting and infecting the

healthy roots of adjacent trees. Armillaria also spreads when

These scattered, small, white patches are Armillaria mellea mycelium

growing beneath bark on the root crown of an avocado tree.

Live infected tree

Dead tree

Tree decline and

death

Spread continues

Primarily spread by

rhizomorphs or mycelial

growth across root contacts.

Common. The Armillaria fungus persists in

stumps, old roots, and wood

pieces in soil. Common.Young tree grows and

roots contact infected

roots, wood, or

rhizomorphs.

Common.Mushrooms

develop during

rainy weather

Young tree

Spores may infect stumps

and tree wounds, spreading

Armillaria root rot.

Not Important.

Figure 21. Armillaria root rot development cycle and spread. Armillaria mellea persists for years in infected roots and wood in soil. New infec-

tions occur when roots grow and contact infected roots or wood or when fungal rhizomorphs grow short distances in soil to contact nearby plants.

Armillaria spores can infect stumps and tree wounds, but spores apparently are not an important source of Armillaria root rot in California.

Eliminating infected trees and removing old stumps, large roots, and wood pieces from soil can break the disease cycle.

Excerpted from Integrated Pest Management for Avocados, ANR Publication 3503

See www.ucanr.org/ipm-avocado

©2008 by the Regents of the University of California

any activity moves soil containing infested wood fragments,

such as during cultivation.

Long after the aerial parts of a tree have been removed,

Armillaria can remain alive in the remaining roots and

stumps. Then, when new avocado trees are planted, the

new roots grow into contact with Armillaria-infected roots

or infested wood pieces and the new tree becomes infected.

Armillaria can also be introduced on infected nursery stock.

Management Guidelines

Provide a good growing environment and proper cultural

practices and use good sanitation to manage Armillaria root

rot. Good drainage is important, as is not irrigating exces-

sively. Armillaria fungus is very susceptible to drying. Citrus

growers sometimes excavate the soil around the trunk to

temporarily air-dry the root crown to prolong the life of an

infected citrus tree. This excavation may also be effective

on avocado trees, but apparently it has not been tested. If

you do expose any root crowns, shade them to protect them

from sunburn. When an infected tree dies, remove it and

any immediately adjacent trees, which may also be infected.

Remove the stumps also, and as many root pieces as possible

from the soil. Thoroughly clean all soil from the equipment

that you use, and leave the soil on-site before you remove the

equipment. Consider replanting the ground with crops that

are not susceptible to Armillaria.

Soil fumigation with chemicals has successfully con-

trolled Armillaria root rot under favorable soil conditions

by preventing spread of the fungus and permitting growers

to replant fumigated areas. Fumigation is expensive and

potentially hazardous, however, and often it is only partially

effective. Consult the latest update of Avocado: UC IPM

Pest Management Guidelines: Diseases (online at www.ipm.

ucdavis.edu) for more information on fungicides and disease

management.

Armillaria can spread by cordlike rhizomorphs (see arrows, top of photo),

which resemble small dark roots. Healthy roots (bottom) are lighter-

colored, usually light brown to whitish.

Armillaria mycelia can develop as large, white, fan-shaped plaques

beneath the bark. After the aerial parts of a tree are gone, Armillaria can

remain alive for years in roots and on pieces of wood in the soil.

JACK KELLY CLARK JACK KELLY CLARK

Excerpted from Integrated Pest Management for Avocados, ANR Publication 3503

See www.ucanr.org/ipm-avocado

©2008 by the Regents of the University of California

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