Oak-Root Fungus Can Kill Trees : Parasites: Recognizing the mushrooms can make it easier to control the fungus, which grows virtually anywhere in the world.
It usually happens on a cold winter day.
You go into the garden, perhaps only to smell the musk perfume of the China lilies, and discover that an old family friend is stricken with an incurable, often fatal disease-- armillaria , or oak-root fungus.
There, at the base of your favorite old shade tree, usually a native coast live oak, is the unmistakable evidence: sinister clumps of a hundred or more honey-colored or light-brown mushrooms of the species Armillaria mellea .
Although oak-root fungus is the common name for this organism of virtually worldwide distribution, it parasitizes a wide range of trees and shrubs. Brazilian pepper, Australian tea tree and Chinese pistache are among its victims, along with the native oaks ceanothus and toyon .
Oak-root fungus produces mushrooms in cool weather following rains, although the mushrooms may not appear at all in warm, dry winters. Learning to recognize them can give you a valuable early warning that you have an infected tree or shrub.
Armillaria mushrooms are easily distinguished by characteristic tight clusters of many mushrooms on wood, although sometimes on buried wood they appear to be growing in soil. As the tawny or yellowish mushroom caps expand to their full size of 2 to 6 inches across, they shed cream-colored spores that give lower mushrooms in the cluster a flour-dusted appearance.
The flesh of the cap and the gills on its underside are typically white, and the tough, fibrous stem is usually encircled by a collar-like white ring.
Oak-root mushrooms, though bitter when raw, are used in cooking. Some fungus connoisseurs rate them as choice, while others find them unpleasantly sour. As with any wild mushroom, extreme caution is advisable.
Don’t eat anything you haven’t positively identified.
Among other mushrooms, the species most likely to be confused with oak-root mushrooms in Southern California is the yellow clustered woodlover ( Naematoloma fasciculare ), which also forms in tight clumps on wood. Poisonous, it is said to be so bitter that no one could eat enough of it to become ill.
Unlike oak-root mushrooms, the clustered woodlover has greenish-yellow gills, yellow flesh, purple-brown spores and no ring on the stem. A deadly poisonous mushroom, Galerina autumnalis , also grows on wood but is much smaller and has fragile stems and brown spores.
Mushrooms are the spore-producing reproductive bodies of the fungus and as such, are merely symptoms of the disease. The role of spores in spreading the fungus is believed to be slight, so there is little to gain from destroying the mushrooms. The real damage is done as the white strands of fungus called mycelia invade and destroy the cambium, the conductive tissue of major roots and trunks of woody plants. As the uptake of water and nutrients is interrupted, the foliage becomes sparse and branches die.
In a badly infected plant, the bark begins to crack and buckle. If the bark is pulled away to reveal the cambium layer, fan-like sheets of white mycelia appear. The mycelia spread through the soil, up to 15 feet a year, invading the healthy roots they encounter.
What can you do about oak-root fungus?
Once there was talk of a fungicidal wonder drug, but those hopes have faded.
Keep in mind that oak-root fungus does the most damage to trees growing under stressful, unnatural conditions.
For coast live oak, a tree perfectly adapted to the Southland’s long dry season, summer watering around the base of the tree is the usual culprit, although insect attacks and the declining vigor of old age can also lead to fungus damage.
For Monterey pine, which resists armillaria in coastal areas, the hot, dry, polluted air of inland valleys is stressful enough to make it succumb to the fungus.
The prospect of losing a fine old oak is painful enough to prompt some gardeners to make heroic efforts. At the Casita del Arroyo demonstration garden developed by the Pasadena Garden Club along the east bank of the Arroyo Seco, an interesting experiment in inarch grafting was attempted to create a new root system for an ailing oak.
The club called in an arborist to graft the root systems of several sapling oaks to the old tree. Two years later, the old oak is still alive and possibly showing slight signs of improvement.
At Descanso Gardens, where summer watering of the camellias planted under native oaks enabled armillaria to spread throughout much of the garden, inarch grafting would be prohibitively expensive.
Instead, superintendent George Lewis fights the fungus by having tree wells dug around oaks and other trees. The wells, as much as 5 feet in diameter and deep enough to expose roots as thick as one’s forearm, allow the base of the tree to stay dry.
Wells are dug around trees whether or not there is any sign of infection, and may slow the progress of the fungus in diseased trees or prevent healthy trees from becoming infected.
An even simpler technique to protect healthy oaks is to adjust or replace sprinklers so they don’t dampen the base of the tree. If you must have azaleas under your oak, plant them several feet away from the trunk to allow good air circulation.
Unfortunately, few gardeners take preventive measures against oak-root fungus, although it is one of the most common plant diseases in Southern California.
David Lofgren, plant consultant at the Los Angeles State and County Arboretum in Arcadia, said he fields many questions about armillaria. They are often stated simply, such as: “Everything in my yard is dying. What could it be?”
By the time symptoms such as yellow foliage, very slow growth or dying branches can be seen, as much as half of a tree’s roots may be dead. If a dead or dying tree is removed, the stump and as much of the root system as possible should be dug out, because the fungus can flourish for decades in deeply buried roots.
Planting a new tree in an area known to contain oak-root fungus is a risky business. Lewis said that in infested soil at Descanso Gardens, reputedly resistant species such as shamel ash and Southern magnolia have succumbed to the disease. Even the supposedly incorruptible coast redwood can be infected with armillaria, but always seems to be able to fight off or outgrow the infection.
Coast redwood has many remarkable characteristics, including very rapid growth and durable wood, but for many gardeners its resistance to oak-root fungus might prove to be its most valuable asset.
