Sally Scalera | For FLORIDA TODAY
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Many residents who grow palms are familiar with the signs of Ganoderma butt rot, Ganoderma zonatum, which is confirmed when a conk is found at the base of infected palms and palm-like plants.
On the other hand, the fungus that can kill most woody trees and shrubs is Armillaria root rot, and it isn’t as well known. Armillaria fruiting bodies, or mushrooms, are typically only produced when there are moist conditions and cooler temperatures.
For our area, that is generally fall, winter and occasionally in the spring. If you have a yard full of trees and shrubs, be on the lookout for Armillaria mushrooms.
Armillaria root rot also goes by the common names mushroom root rot, shoestring root rot, and honey mushroom rot. This disease decays the root system of many woody trees and shrubs.
This genus of fungi can be found world-wide, from the warmer tropics to the colder temperate regions of the north. Armillaria has a large list of hosts that it can infect, including many hardwoods and conifers.
The species within this genus are grouped into two categories that include either primary pathogens that can attack and kill healthy trees, or opportunistic pathogens that kill only unhealthy or stressed trees. The most common species of Armillaria found in Florida are A. tabescens and A. mellea, which fall in the opportunistic category that typically attack plants that are stressed.
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Some of the stressors that can pre-dispose plants to pathogens include drought, cold injury, disease, pest infestations, planting too deeply, over pruning, damaged roots and improper fertilization. The fungi can spread from plant to plant, by both the movement of rhizomorphs (which are thick mycelial growths that transport nutrients), the release of spores from fruiting bodies and root-to-root contact.
A damaged root system is susceptible to infection, whereas healthy roots can resist infection.
Because of the large number of woody plants that can be infected by Armillaria, it is not uncommon to find groupings of trees and shrubs in an area all infected by Armillaria.
Symptoms of Armillaria root rot include wilting, sparse foliage, overall decline, dead branches, branch or trunk failure, dieback, stunted growth and death.
Symptoms alone cannot be used to determine the cause of the plant’s decline, because there are many other potential causes of these symptoms, ranging from cultural practices to pathogens. It can take one to three years after the infection has occurred before symptoms begin to appear.
During storms, trees compromised by Armillaria may fall over because of a weakened trunk base (butt rot) or a decayed root system.
The only true sign (where you see the actual organism) of Armillaria root rot would be the presence of the mushrooms, which is also true for Ganoderma butt rot of palms.
The mushrooms of Armillaria are clusters of short-lived, yellowish to honey-brown mushrooms. The mushrooms can be found at the base of an infected trunk, on the stump of a dead tree, or away from the trunk growing out of an infected root. The individual mushrooms have white gills when viewed from below. The discovery of Armillaria mushrooms is all that is needed to confirm the disease.
When it comes to managing Armillaria in the landscape, the best strategy is prevention. This can be accomplished by creating a healthy, living soil around the root systems of your trees and shrubs.
Since Armillaria is a soil-borne fungus, protecting the root system from damage is also very important. The most beneficial action that can be done to protect woody plants from a soil borne fungus is to inoculate the root systems with beneficial mycorrhizae. They form a symbiotic relationship with most plants and all woody plants.
Trees need mycorrhizae so much that they are considered mycorrhizal dependent. Plants share sugars, as root exudates, to attract and feed the beneficial bacteria and both endo and ectomycorrhizae. In return, the microbes provide the plant with the nutrients that it requests.
A thriving soil food web creates an aerobic soil, whereas pathogens can only survive in anaerobic soils. Also, ectomycorrhizae create a physical barrier that protects roots from infection.
For more information on how to add life to the soil, email me as sasc@ufl.edu.
Once a tree or shrub is infected with Armillaria, it is best to remove it as quickly as possible. There are no fungicides that can be used against Armillaria to save a plant once it has been infected.
Stumps should also be removed so they won’t be a food source for the fungus.
Strategies that can be used when planting a new woody plant include planting Armillaria resistant species and inoculating the planting hole with a granular mixture of beneficial bacteria and mycorrhizae.
For more information on Armillaria, and the list of resistant trees, refer to our bulletin at edis.ifas.ufl.edu. Search for “Armillaria.”
Now that you know what to look for, go out and walk around your yard to make sure there are no Armillaria mushrooms. While you’re at it, be on the lookout for Armillaria as you walk the neighborhood or visit family and friends.
Identifying Armillaria and being proactive are great ways to protect landscapes from an opportunistic destroyer of most trees and shrubs.
Sally Scalera is an urban horticulture agent and master gardener coordinator for the University of Florida’s Institute of Food and Agriculture Sciences. Email sasc@ufl.edu.
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