Honey fungus is a problem because it has the potential to infect thousands of plants and crops, including large areas of natural forest. There is no registered control of this pathogen, and, once it takes hold, it is very difficult to eradicate. Often, trees and plants must be destroyed to control this parasitic fungus.
By being aware of honey fungus and recognizing the effects of this fungal root rot, you may be able to do something about it before it’s too late. As you read, imagine the “red weed” as described by H.G. Wells in the fictional novel The War of the Worlds and so graphically depicted in the movie.
The Short Answer
Honey fungus is the common name for a genus of fungi called Armillaria. This disease-producing pathogen causes white rot in trees, shrubs, woody climbers, and some woody perennials. It can usually be found at the base of plants and will eventually kill off the roots by causing rot between the bark and the trunk.
The Long Answer
Clearly, honey fungus is not a harmless, sweet mushroom like its name suggests. Let’s delve deeper into the biology of this pathogen, how it spreads, and how to prevent it.
What is Honey Fungus?
Initially called Armillaria mellea, this is a parasitic organism with varying species.
Honey fungus was originally identified in the 1800s. It grows extensively in North America and Europe, but some other continents also show signs of the fungus. Initially, scientists believed this fungus was only the Armillaria mellea species. However, several more species of Armillaria have been discovered and also fit under the common name “honey fungus.” There is plenty of research still to do on the subject, as the species differ across the world. They all share a parasitic nature and the ability to kill their woody plant hosts.
The world’s largest living organism is an Armillaria species (ostoyae), first discovered in 1998 in eastern Oregon. The fungus covers nearly 965 hectares of land. According to scientists, it could also be the oldest living organism.
Identification
Identifiable by yellow caps, this mushroom may be edible but requires caution.
The below-ground fungal parasite regularly produces fruiting bodies (mushrooms) above the soil surface. If fruiting, it can be identified by its yellow cap mushroom growing at the base of infected trees and other woody plants.
The name “honey” is because of its color and not the distinctly acidic taste. Some people consider the mushrooms edible, while others warn against eating them raw, suggesting they are only safe when cooked. Like all wild mushrooms and foraged foods, you should use reputable guide books to clearly identify the fungus before consuming it.
The caps of the mushrooms can be between two and six inches wide in colors ranging from honey yellow to reddish brown. They have pale cream gills underneath with dark reddish spots when they mature. The stems are three to six inches long, with a distinctive stem ring forming near the top.
How it Spreads
It spreads via spores and mycelial threads, infecting plants through roots.
Honey fungus spreads in two main ways – through spores and mycelial threads.
Basidiospores are discharged from the mushrooms and dispersed by wind, plants, or animals. When the spores land, they may grow on dead wood stumps, leaf mold, felled trees, etc., spreading throughout a forest.
But the most infectious way this parasite spreads is by mycelium. Mycelial threads are also called rhizomorphs or “bootlaces,” and can spread up to 10 feet long to find a new host. The fungus spreads underground through the soil and attacks the roots of new plants. It then spreads through the infected roots to other plants in the vicinity through the roots and expansion of the rhizomorphs. Any healthy plant in its path, whether mature or a seedling, can be infected.
How it Kills
Rhizomorphs cause extensive root damage, leading to tree death.
Once the rhizomorphs have taken hold, they can cause extensive root damage. Sometimes, the fungus spreads around the tree, effectively ring-barking it. An infected tree does not have much chance of survival. The root rot may take a while to see visibly in the plant, or it may kill the plant quickly.
What Plants it Infects the Most
This fungus infects a wide range of plants, some being more susceptible than others.
Conifers, broad-leafed trees, fruit trees, woody shrubs, and perennials are susceptible to this parasite. From the smallest seedling to the largest forest tree, most woody plants can be infected. This is an equal opportunity parasite.
There are no known plants that are completely immune to this root rot fungus, but there are those that are more prone to getting the disease. These include peaches, pepper trees, apples, pears, privets, beech, birch, hydrangeas, and magnolias.
What to Look for Above Ground
Mushrooms appear mainly in autumn, targeting stressed or disturbed plants.
Autumn is the main season to spot the mushrooms above ground.
Trees or plants that are stressed are more susceptible to attack. Areas where the natural vegetation is disturbed or cleared are especially vulnerable to infection.
Infected trees and plants will sometimes exhibit dieback, a lack of new leaves, or paler-colored leaves in spring. You may also see the upper plants of the plant die back, especially in hot, dry periods.
Just before a plant dies, two opposing signs could happen: They may stop flowering, or they could exhibit a massive flowering period with loads of fruit.
Some plants will bleed sap and crack, have insect infestations caused by a stressed plant, or start losing their bark at the base of the plant.
What to Look for Below Ground
Rhizomorphs spread near the soil surface, forming parasitic threads between bark and stems.
The mycelial threads or rhizomorphs are black threads that form a structure reminiscent of aliens. This alien-looking root system spreads along parallel lines with side ‘roots’ spreading across the lattice. This lattice will move through the soil through the top eight or so inches . They like to remain close to the surface of the soil.
The rhizomorphs also form a pattern of parasitic threads between the bark and the stem of trees and plants. Sometimes, the only way to identify it is by removing some of the bark at the base of the plant.
Dead or decaying roots will often have a very strong mushroom smell.
Control Options
These are some of the control and preventative options to consider if you find this parasite in your garden.
Prevention
Prevent the fungus by ensuring plants aren’t stressed. Provide proper watering and drainage.
There are not many forms of prevention that will work on this parasite, but reducing plant stress will go a long way in discouraging fungal diseases or pests in general. Make sure the soil is well-draining and not compacted.
Also, don’t damage or waterlog the roots. They need air to perform their very important duties of absorbing water and nutrients, and will not be able to do so if their roots are drowning. Waterlogged, compacted soil creates a space where root rot fungi can easily colonize your plant or tree.
Chemical Control
There is no chemical control for this disease; remove infected plants and burn roots to eradicate.
There is yet no effective chemical control or registered fungicides for the control of honey fungus. It is recommended to remove an infected plant and burn it immediately, including the roots and any stumps.
By destroying its food source, you will destroy the fungus. If you’re unsure whether or not you should do this, consult your local extension office.
Biological Control
Trichoderma, a beneficial fungus, may help control honey fungus; physical barriers limit spread.
Although there is nothing registered for the control of honey fungus, there are some options available. For example, a naturally occurring ‘good’ fungus called Trichoderma is a plant protector. In some studies, it was found effective for the control of some Armillaria species. Trichoderma is found in mulches and woody plant materials, but it is also available as a product that can be added to stimulate plant growth and help protect the roots.
Some control can be achieved by limiting the spread of the mycelial threads. Do this by placing physical barriers in its way, like a heavy plastic barrier buried around the plant to at least 18 inches deep. Also, if you break up the wood bits regularly, it will slow the spread of the fungus.
Final Thoughts
It’s somewhat unsettling to know that an underground network of parasites can pounce on any unsuspecting tree or plant. However, there are also thousands of beneficial soil fungi that can keep your plants happy and healthy.
The honey fungus is another reminder of the power of nature. We can overcome this pathogen by being responsible gardeners and paying close attention to our plants. A healthy, well-tended garden is not often infected by destructive pathogens, but the more we know about them, the better we can prevent them.
