IMPORTANCE: Annosus root rot (ARR)
is a commercially important disease of all
conifers. Loblolly and slash pine are most
severely affected. It typically enters the
tree through a wound, grows through the
heartwood into the roots, and causes decay
in the root system. Diseased trees are
subject to windthrow and breakage.
IDENTIFICATION: The fungus Heterobasidion annosum causes
ARR, formerly called Fomes annosus. A fungus growth extending from the trunk
of the tree is a “conk.” Conks are positive ARR identification. When fresh,
conks are tan to brownish on the upper surface and white with tiny pores on
the lower surface. Colors darken once the conk passes through winter. They
are rubbery and difficult to tear. Conks are often at the base of dead or
dying trees, stumps, or under root masses of windthrown trees. In trees with
deep litter, conks may appear on the north side where humidity is higher. In
Alabama, most conks develop from December through March. Insects may destroy
them before summer or fall.
Not every infected tree will have a conk and in some infected stands,
there are none. This is common in stands that have been burned. If no conks
are found, growing fungus from suspect trees in the laboratory can identify
Annosus root rot symptoms can be confused with littleleaf disease. In the
absence of conks, remember that littleleaf disease usually occurs on poorly
drained clay soils rather than well-drained sandy soils where ARR is
commonly found. Bark beetle infestations are often associated with ARR.
After identifying ARR in a few stands, strongly suspect the disease in
other stands if there is some combination of the following:
- Pine stands with dead and dying trees often in clusters or rows.
- Trees leaning or blown over from lack of supporting roots.
- Stringy white rot of wood in roots and/or butt.
- Sparse crowns with off-color needles, often with abundant cones.
- Resin-soaked root areas with discolored, dead, or rotted end
- Mortality in second or third year following thinning and continuing
for several years.
- Pine stands infested with southern pine beetles or Ips bark beetles.
ARR SOIL HAZARD RATING: In Alabama, any pine or pine
stand may have ARR, although vigorous stands on suitable sites may suffer
less damage. A high hazard site is one on which ARR can be reasonably
expected to cause mortality and growth loss of a value greater than the cost
of prevention. The most consistent indicator of high hazard sites is
well-drained, sandy soil to a depth of at least 12". These soils consist of
70% or more sand. Organic soils and soils with indicators of poor internal
or external drainage are a low hazard. A high ARR hazard site will always
remain so and will always require careful consideration. Every soil type in
Alabama has an ARR hazard rating. County foresters can provide this
information. A TREASURE Forest plan will also contain this information along
with recommended prescriptions.
PREVENTION: To prevent ARR, maintain healthy stands and
recognize high hazard sites. Trees planted out of their natural range on
high hazard sites are more susceptible. Longleaf pine is slightly less
susceptible on high hazard sites. Planting on wider spacing and thinning
should help avoid widespread infection. ARR typically enters the stand when
fungal spores land on freshly cut stump surfaces. The fungus grows into
nearby live trees via root grafts or contacts. To prevent ARR, treat stumps
with borax whenever thinning in a high hazard area.
CONTROL: Once ARR is established and substantial mortality
is occurring, control is necessary. Thinning is very risky because root
damage promotes infection and the residual stand will be expected to
deteriorate rapidly. The stand should probably be clearcut, unless losses
can be absorbed until remaining trees reach sawtimber size. The higher sale
price may make up for the loss. The site may be regenerated in pine witha
small mortality percentage. The problem usually becomes apparent a few years
after a thinning cut, often when trees are close to sawtimber size.
If the stand has a localized group of infected trees, salvage the trees
including a buffer strip of green trees as wide as the average height of the
dominant trees, and treat the stumps with borax. If southern pine beetles or
Ips bark beetles are present or are a potential hazard on high ARR hazard
sites, their prevention or control must be reconciled with recommendations
THINNING ON HIGH ARR HAZARD SOILS: Always cover freshly
cut stumps with at least 1/8” of borax within 24
hours of harvest. Borax (Sodium Tetraborate Decahydrate) is sold under
several names: Borax, Sporax, Twenty Mule Team Borax, etc. The best method
is to sprinkle material “salt-shaker” style. These applicators are available
in one or two pound sizes. At proper rates of application, one pound of
product will cover 50 square feet of stump surfaces: 260 six-inch stumps,
158 eight-inch stumps, 80 ten-inch stumps, or 60 twelve-inch stumps.
Photo Credit: USFS
Eastern Forest Environmental Threat
Forest Encyclopedia, USDA Forest Service
Forest Health Protection-USDA