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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