What is Armillaria mellea and why is it so dangerous for plants?
Armillaria mellea, popularly known as honey fungus o white rot, is one of the most feared phytopathogenic fungi in agriculture and gardening. This fungus, belonging to the basidiomycete group, is known for its destructive action on a wide range of woody species, including fruit trees, ornamental plants, and forestry. Its danger lies in its ability to cause root rot and decay, leading in many cases to irreversible death of plants.
The peculiarity of Armillaria mellea It acts as a saprophyte on dead wood, but also as a pathogen on living roots, allowing it to survive long periods in the soil and infect new plants when conditions are favorable.
The life cycle of this fungus includes the formation of honey-colored fruiting bodies (mushrooms), often visible at the base of affected trunks during the fall. However, much of its development remains hidden in the soil, where the rhizomorphs (black, threadlike root-like structures) spread in search of new food sources, actively spreading disease.
The importance of knowing and detecting the Armillaria mellea It is capable of completely eliminating the roots of trees, vines and shrubs, severely affecting the productivity and longevity of crops.
Detailed characteristics of Armillaria mellea
From the point of view mycologicalArmillaria mellea is characterized by having:
- Convex or flattened hat, honey color, with yellowish tones and small brownish scales.
- Adnate or subdecurrent blades, clear in young specimens and yellowish with age.
- Foot generally long, curved and fusiform, with a persistent membranous ring.
- Spores of white.
- Whitish and firm flesh in the cap, woody and fibrous in the stem.
One of the most striking curiosities of this fungus is its ability to produce bioluminescence, that is, visible light thanks to biochemical processes in the mycelium.
The edibility of the mushroom is controversial: although it is consumed in some cultures, it is important to know that some people are intolerant to it, and ripe mushrooms can be bitter and unpleasant, with a strong odor.
In its vegetative phase, Armillaria mellea It develops dark brown, shiny, corded rhizomorphs, which constitute the main underground propagation structure. The mycelium also forms whitish plaques under the bark of roots and trunks of infected trees.
Propagation and development conditions
The mushroom survives on woody debris and dead wood, where it can remain for many years and from there infect new trees or shrubs, even if these were planted long after the original affected plant was removed.
La propagation It is produced mainly through:
- Underground rhizomorphs that advance through the soil and infect healthy roots upon contact.
- Spores produced by fruiting bodies, which colonize dead wood debris and form new foci of infection.
- Pruning and work tools contaminated with infected parts.
The environmental conditions that favor the development of Armillaria mellea include:
- Land compact and heavy, with poor drainage and a tendency to waterlogging.
- Areas with buried woody debris or very high humidity.
- Low aeration and young plantations on old forest land.
For these reasons, the disease can occur in both new and mature plantations, and the rate of spread is usually greater in young and weakened trees.
Host plants and crops affected by Armillaria mellea
Armillaria mellea has a very wide range of hosts, affecting:
- Fruit trees such as almond, pistachio, walnut, peach, cherry, persimmon, apricot, plum, apple, and papaya trees.
- Vines, rose bushes, kermes oak, grapevines, and other ornamental shrubs.
- Mountain trees such as pine, ash, carob, myrtle and boxwood.
The sensitivity and appearance of symptoms may vary depending on the species, but the almond tree, pistachio tree and vines They often show a special vulnerability, suffering significant losses in both quality and quantity of production.
Symptoms of the disease: How to identify Armillaria mellea
Detecting Armillaria mellea infection is not always straightforward, given the nonspecificity of the initial damage in the aerial part. Some indicative symptoms include:
- Progressive loss of vigor: reduced growth, wilting and lower yield.
- Yellowing and premature fall of the leaves, especially on branches that seem to dry out for no apparent reason.
- Presence of honey-colored mushrooms at the base of the trunk during autumn or other wet seasons.
- Root decay: blackening and damp rot with a moldy odor.
- Whitish plates of mycelium and brown-black rhizomorphs under the bark of the roots and near the collar of the tree.
- Stand formation: Damage spreads along the ground, affecting adjacent trees, forming circular or irregular areas of dead plants.
- Exudation of sap or gum at the base of the trunk in some species.
Internal lesions have a fibrous texture, brown or black color, and when the bark is lifted, fan-shaped mycelial masses are revealed. When the tree is severely affected, death is inevitable, and the soil is infested for future plantings.
Diagnosis and monitoring of the disease
Because the airborne symptoms are not exclusive For Armillaria mellea, a systematic inspection is essential. Diagnosis includes:
- Remove the base of the trunk and visually examine suspicious roots and the neck of the tree.
- Look for white plates and rhizomorphs under the sloughed bark.
- Confirm the characteristic moldy smell and wet rot.
Regular monitoring of plantations is key to detecting outbreaks early and preventing the disease from spreading to other plants.
Infection prevention and cultural practices
Prevention is the main defense against Armillaria mellea, given the difficulty of eliminating the fungus once it has established itself in the soil. Among the most recommended measures are:
- Select healthy, well-drained soil, avoiding soils where affected plants or excess moisture previously existed.
- Use certified and pathogen-free plant material in new plantations.
- Practice a crop rotation, avoiding planting sensitive species in risky soils.
- Disinfect pruning tools and work between plants to avoid spreading the fungus.
- Remove and destroy all woody debris and diseased plant roots.
- Perform pruning during dormant periods and apply preventive treatments before and after the intervention.
- Avoid waterlogging, improve drainage, and adjust watering and fertilization to avoid weakening the root system.
- In contaminated soils, delay replanting and deep till the land and remove old roots.
- If available, plant resistant species around the most susceptible trees, since some roots emit compounds that hinder the spread of the fungus (such as boxwood, ash, myrtle, Aleppo pine or carob).
Armillaria mellea treatment: curative and preventive strategies
At present, There are no fully effective curative treatments for Armillaria mellea once the disease is established. However, different methodologies can be applied to:
- Reduce the spread by physically eliminating sources.
- Apply compounds that stimulate the plant's resistance and hinder the spread of the fungus.
- Improve tree physiology to facilitate the recovery and synthesis of new vascular tissue.
Healing Program
When the first symptoms are detected, action must be taken quickly and forcefully. The healing program consists of:
- Soil applications through the irrigation system with fungicidal products and root activators.
- Carry out intense and frequent foliar applications, interspersed with soil applications.
- Delimit the plot, marking deep trenches that limit root expansion and improve drainage.
- Treat the entire plot if the infection is advanced, given the speed at which the fungus spreads underground.
- Supplement with metabolic regulators and plant-based enzyme packs to enhance the formation of new tissue.
It's fundamental associate these treatments with cultural management measures and soil improvement, as well as the removal of any severely affected plants along with their roots.
Preventive Program
Once the spread of the fungus in the plot has been controlled, it is recommended to start a preventive program consisting of:
- Periodic applications to the soil and via foliar application with low or moderate doses of fungicidal products, aimed at keeping pathogen levels low.
- Post-harvest maintenance treatments, especially to protect dry or winter pruning wounds.
- Use of compounds with high persistence and penetration capacity to ensure the protection of vulnerable tissues.
These methods seek prevent new outbreaks and limit the reactivation of dormant rhizomorphs or spores in the soil.
Importance of early detection and limitations of treatment
It should be noted that the effectiveness of treatments decreases dramatically when the disease is very advanced. If root degradation is severe and the imbalance between the above-ground and root portions is very marked, the plant will inevitably die.
In initial phases, notable improvements in root development can be observed, with an increase in viable branches and greater efficacy of subsequent treatments. However, prevention and integrated management are the best bets for avoiding the appearance of Armillaria mellea or its proliferation in plantations.
