By Mary Bernard, Master Gardener
Armillaria root rot, also known as oak root fungus, is one of the most
widespread plant diseases in
Armillaria can develop slowly, and symptoms may not appear until the fungus is
well established. Affected trees usually
show a general decline in vigor over many years. Sometimes trees that look healthy will
suddenly wilt and die in a matter of weeks.
The above ground symptoms are similar to other root problems, including
too much water, Phytopthora root rot, or gopher damage.
The key symptom to look for is trees that are declining in patches and the
patches seem to get larger each year.
Roots infected with oak root fungus have white to yellowish fan-shaped
mycelium between the bark and the wood.
Dark brown to black structures that resemble shoestrings sometimes can
be seen on the root surface. Sometimes,
large, densely packed, honey-colored mushrooms form at the base of infected
trees in late fall/early winter after rains.
To confirm the presence of Armillaria, dig around the crown of the tree and
scrape off the bark on small sections of the crown and main roots. It should be easy to see the dense, felty
tissue between the bark and the wood.
Infected wood will have a strong mushroom smell and feel slightly
spongy. Sometimes it may be difficult to
locate the mycelium in the earlier stages if the infection has not yet moved up
to the crown.
There are no effective fungicide treatments for the control of the disease in
living trees. The fungus can survive for
many years in the dead or living tree roots.
Remove as many roots from infected soil as possible before replanting.
Armillaria is sensitive to drying and grows most rapidly under wet
conditions. For this reason, heavy
watering should be avoided. Air-dry the
soil before replanting. Physical
barriers such as root collars can contain infection centers. Prepare a new landscape site well before
planting and provide proper cultural care.
Prevent healthy roots from coming in contact with diseased ones, thus
avoiding spread of the disease. Plant
only Armillaria resistant species in locations where oak root fungus has been a
problem. Call the UC Cooperative
Extension office for information on plants resistant or susceptible to
Armillaria.
(Note: Although another oak killer - Sudden Oak Death - has received a lot of
public attention this year, it has not yet been detected in