Felled By Tiny Fungi
Urban trees tend to have shortened lives, some living no more than 50
to 80 years. Urban forests in many metro areas have started to mature
and decline, and are very susceptible to trunk-rotting and buttress
Wood-rotting organisms can slowly nibble away at trunks and buttress
roots. Trees often regenerate new, nonstructurally supportive feeder
roots that mask the signs of structural root loss. Many trees that
topple look perfectly healthy before they fall. Afterward, it becomes
clear that there were absolutely no structural roots remaining for
The best time to scout for symptoms of a fungal infection is just after
a long period of cool, wet weather.
organisms like these conks often appear on trees after a long period of
cool, wet weather. Image credit: April Sorrow
The "root collar" is where the roots meet the tree trunk. This area is
critical in its function as the main link between the upright trunk and
Trees are load-bearing structures and are designed to support great
stress. Trees operate under the same physical principles of weight
distribution as skyscrapers. Stress from wind on the aerial portion of
the structure is transferred down to the foundation.
Just as buttresses on medieval churches are an architectural feature
designed to support the walls, the buttress roots of the root collar
are designed to support the tree. The decay of root collar wood reduces
the structural integrity of the tree.
Adventitious roots are roots that arise from latent buds in wood in
response to stress. As a tree slowly loses its main roots, it makes new
feeder roots. Over the course of years, a tree can supplement root loss
due to rot by creating these new, nonsupportive roots.
The most common buttress and root collar-rotting fungus of water oaks
is Inonotus, Inonouis dryadeus or "the weeping conk." It mostly affects
oaks, but can affect conifers, too.
Inonotus enters trees when a lack of tree vigor weakens the tree. In
oaks, rot doesn't occur much above ground level. Massive fruiting
bodies start out as large, white, softball- to bowling-ball-sized
"marshmallows" that ooze clear, yellow liquid containing spores. Wind
and rain spread the spores in November and February.
Scouting for Inonotus requires a sharp eye. After the "marshmallows"
dry, they turn into woody structures that are difficult to discern from
tree bark. The fruiting bodies can be seen when they are fresh, but
once they dry, they become camouflaged. The fruiting bodies are
perennial and grow to astonishing sizes.
|Ganoderma root rot
One of the most common buttress rots of Southern hardwoods is also one
of the most beautiful native fungi of Eastern forests. Ganoderma
lucidum is a fungus with fruiting bodies found on buttresses or exposed
roots. It is active, aggressive and can seriously undermine tree
Affected trees usually show rapid decline. The symptoms include
shortened twig growth, off-color foliage and branch dieback. Signs of
the disease first appear as reddish-colored varnished stalks emerging
from the soil around infested trees. These fruiting bodies eventually
flatten out on the top into a half moon-shaped bracket.
Ganoderma invades physically damaged trees. Soil compaction,
lawnmowers, vehicles, herbicides and other damage can expose trees to
infection. Spores infect wounds and fungus spreads through the root
collar and into roots, and it can spread through root grafts.
Armillaria mellea, or the "shoestring root rot" or "oak root fungus,"
is an aggressive tree pathogen. It causes 35 percent of tree deaths in
North America. It is easily recognizable when it is fruiting by big
bunches of 12-inch-tall, honey-colored mushrooms that grow from shallow
roots or the root collar.
Armillaria causes reduced tree growth, undersized chlorotic leaves and
death. The fungus attacks in conjunction with other pests, like
wood-rotting fungi in the main trunk and wood-boring insects. It kills
trees and makes them more susceptible to invasion by buttress-rotting
Inonotus, Ganoderma and Armillaria are preventable but not curable.
Stressed, damaged and weakened trees are more susceptible, so
prevention is key.
Avoid planting trees in undersized spaces. Consider the ultimate size
and choose trees that will not overgrow their space. Do not plant trees
so close to structures that the roots will become restricted.
Use care to prevent trees from suffering wounds, which become the
entryways for fungi.
Finally, care for mature trees properly and try to avoid poor tree
vigor. Symptoms of decline include leaf drop, bark shedding, limb
falls, poor leaf production, and size and cavities in the trunk.
Proper fertility and water management will do more for tree health than
If you find buttress-rotting fungal conks on your trees, call an
International Society of Arboriculture-certified arborist and ask to
have the tree evaluated for safety.
Pettis, University of Georgia Cooperative Extension agent in Rockdale