Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: oak root fungus

After Flooding, Buried Trunks a Problem

A trunk "below grade", that is a buried trunk, is a problem for most trees.  Willows and other riparian trees along water ways that are inundated regularly can adapt to a change in soil depth around their trunks.  That's not true of most of our commercial tree species, and avocados and citrus are really susceptible to buried trunks/stems. they asphyxiate.  This can be a very common problem at planting when a hole is too deep and the new tree settles in the loosen earth and gradually the stem is buried.  Or, when the grower is doing the "right thing" and using an organic planting mix that gradually decomposes and the tree settles into the ground and the crown is covered by dirt.  The tree then starts looking bad -  leaves yellow.  The canopy defoliates.  All the while, the grower is putting on more water and more fertilizer and the tree looks worse and worse.  Remove the soil from around the base, and voila, in a few months the tree is happy again. If the dirt isn't moved, the stressed tree is now susceptible to root rots, both Phytophthora and Armillaria.  Those problems are a lot worse than just lack of air.

This suffocation is a common problem after flooding.  Dirt from higher up moves into lower positions, gravity moving dirt can move a lot.  It accumulates around the base of the tree.  The grower is preoccupied with other things that occur with flooding and does nothing.  A few months later, the trees start turning yellow especially when the weather warms up and they are more active and more water is being applied.  Flooding can also spread disease organism from other areas that are contaminated.  Oak root fungus is frequently spread in flooding waters, especially in the lower positions in little valleys.

Pulling that accumulated soil away from tree is important for tree health. It's something that needs to be done soon after the flooding incident, or any event that buries the tree trunk.

buried tree trunk
buried tree trunk

Posted on Friday, January 12, 2018 at 6:44 AM

Name the Biggest TERRESTRIAL Organism

Mississippi State University

Amanita muscaria fungus growing in a high elevation conifer forest in Hawaii. We can see much of this mushroom above ground, but it has an incredible structure under ground as well. Credit J Meulemans

Have you ever thought about it? I'm sure a lot of you have…what's the biggest living thing on earth? If you're like me, you immediately think of animals like the blue whale. In this case, we're just talking about organisms that spend their life predominantly or entirely on land. Anybody thinking of the rhinoceros, or maybe the African elephant? Nope!

The answer to this question might surprise you. The largest terrestrial organism on the planet is actually a fungus! Not your typical, garden-variety mushroom, but a fungus, nonetheless. And while it is edible, it comes with a few problems. Its common name is honey fungus.

The soil is filled with living organisms, large and small. Bacteria are the most numerous soil organism – there can be more than a million in a teaspoon of soil! There are over 10 trillion types of soil bacteria!

There are “only” 100 billion types of soil fungi, but they play in important role in the life cycle within the soil. Fungi break down decaying plant material into nutrients that other plants can use to grow. They soak up water in water-logged soils. You might be familiar with seeing mushrooms growing on moist logs, or in soil with wet leaves.

fir tree needles

Fir trees were particularly hard hit by the effects of honey fungus. Credit: Böhringer Friedrich https://commons.wikimedia.org/w/index.php?curid=10391442

Honey fungus is a bit different from many other fungi. It's not one that landowners will welcome because it can kill trees. In fact, it gained fame in the late 1990s as the “culprit” that was killing fir trees in the Pacific Northwest. (Fir trees are widely sought after as beautiful landscape plants,  Christmas trees, and for lumber used in building homes.)

Honey fungus is a parasite – Armillaria solidipes. Parasites are organisms that live off other organisms, sometimes hurting them. Honey fungus is widely distributed across the cooler regions of the United States and Canada. It is very common in the forests of the Pacific Northwest.

These fungi grow in individual networks of above and below-ground fibers called mycelia. Mycelia work in a similar fashion to plant roots. They draw water and nutrients from the soil to feed the fungus. At the same time, they make chemicals that are shared with other organisms in the soil. Sadly for the fir trees, the honey fungus can kill already weakened or stressed trees. The natural action of this fungus can be destructive in forests, leading to widespread die-off in timber stands.

One cool feature of the underground network of mycelia of all fungi is that they help hold soil particles together. Just like plant roots, mycelia work to prevent soil particles from blowing away in the wind or being taken away by running water. They are a very important part of the soil ecosystem.

Scientists have shown that Armillaria solipipes identify and connect to each other. That's right: when mycelia from different individual honey fungus bodies meet, either in or on the soil surface, they can attempt to fuse to each other. The fungi must be genetically identical honey fungi. When the mycelia successfully fuse to each other, they link very large fungal bodies together. This, in turn, changes extensive networks of fungal “clones” into a single individual.

The largest honey fungus that has been identified in North America is located in Oregon. It measures 3.4 miles across! Scientists also believe that this particular honey fungus may be over 2,000 years old. The next largest honey fungus is in the neighboring state of Washington.

While honey fungus is impressive in both age and size, it isn't always a favorite of scientists and landowners because of its parasitic nature. But it is the largest organism on earth, and scientists have had only a few decades of research to understand how mycelia fuse and communicate. Perhaps one day, research discoveries about honey fungus could lead to a new medicine (think Penicillin), or new ways to grow food – the possibilities are endless!

And this is the largest terrestrial organism. Who knows what lurks in the depths of the ocean?

P.S.

Armillaria solipipes is one of the many species that was once lumped as Armillaria mellea, oak root fungus.  So many woody perennials are susceptible to this fungus.  It turns out that avocado for whatever reason tends to be somewhat more tolerant of oak root fungus than many other trees.  Under stress, it too will succumb, though.

Amanita
Amanita

Posted on Friday, January 5, 2018 at 6:44 AM
  • Author: Jesse Morrison
Tags: armillaria (5), avocado (287), citrus (334), melea (1), oak root fungus (4)

Armillaria or Oak Root Fungus

Armillaria Root Rot

There have been a lot of new avocado orchards planted during the last few years. These often have been in old ‘Valencia’ orchards or lemons that had poor production. In order to save money, growers have just cut the trees at ground level and replanted the avocados near the stumps. Avocados have recognition of being resistant to Armillaria, but in this environment of high disease pressure, they can fail.

Armillaria root rot is common, yet is an infrequently identified and poorly understood disease. It is capable of attacking most species of trees and other woody plants growing in California. It is sometimes called “shoestring root rot” and the causal fungus is often referred to as the “honey mushroom.” Because oak is one of the preferred hosts, it is alsocalled “oak root fungus.”

If a tree undergoes a slow to rapid decline without any obvious reason, suspect Armillaria as the cause. Certain areas, such as drainage areas from chaparral or woodlands are likely areas for this disease. Old roots left underground provide a food base for continued fungal growth and survival.

General symptoms of Armillaria resemble those of other root disorders. These symptoms are disrupted growth, yellow foliage, branch dieback, and resin or gum exudates at the root collar. Trees may die rather abruptly without showing any decline symptoms. Avocados typically have a rather protracted death, but in citrus it can be rapid. Only rarely can the disease be diagnosed without examining the larger buttress roots and root collar of the tree. After carefully removing the soil, examine forthe presence of:

1) Rhizomorphs, or fungal ‘shoestrings’ attached to the wood under the bark. These may occur beneath the bark for some distance above the soil line in advanced cases, rarely they may radiate from the wood into the soil. Rhizomorphs may also grow out from the larger roots, resembling feeder roots in appearance. They are about the diameter of pencil lead and vary in color from black to reddish brown. The interior consists of white mycelial tissue.

2) Decayed areas of wood at the root collar or on the crown roots. Armillaria causes a white rot and the wood develops a stringy texture. Roots in advanced stages of decay may be soft, yellowish and wet.

3) Veined, white mycelial fans between the bark and wood where the cambium has been killed. Sometimes this fan (or fans) extends quite far above the soil line beneath the bark.

4) Soil remaining attached to theroots.

5) Characteristic mushrooms on the lower trunk or on the ground near the infected roots. These shortlived annual fruiting structures of the disease-causing fungus may develop during the fall or winter rainy season and may occur in small clusters or in large numbers. The stalk is typically yellow and 3 inches or more long. Usually a ring is connected to the stalk just below the cap. The cap is 2-5 inches across and often honey-yellow. It may be dotted with dark brown scales. The underside is covered with loosely spaced white or yellow gills radiating from the stem. After the disease has been identified, the grower should study the situation to determine the role Armillaria root rot has played in causing the decline or death of the tree. Frequently the fungus is only involved in a secondary manner by invading and destroying roots after the tree has been exposed to stress of some form, such as severe drought, water logging, or soil fill over the roots. The fungus can also act as a saprophyte feeding on dead wood. It is frequently involved in the decay of old tree stumps and roots.

Many oaks are lightly infected with the disease for years with no resultant damage except for isolated pockets of buttress root rot which are walled off by the tree and have no ill effects. Other infected trees show no damage until subjected to stress. Accumulating evidence suggests the type of root exudate that is produced influences the susceptibility of the tree. Certain forms of stress cause a shift in exudates that promote rapid development of the fungus and may hasten tree invasion and decay.

Spores are produced by the mushroom fruiting structures (mushrooms) and disseminated by air currents and introduced into new area. Once the fungus enters the cambium and bark tissues, mycelial fans develop during the parasitic phase of the attack. Subsequently, mycelium invades and decays the woody tissue of the roots and sometimes also the base of the trunk. Under proper conditions the fruiting structures form at or near the base of the infected tree, completing the life cycle.

Direct control of the fungus in a diseased tree is not possible with present technology. However, in many instances the fungus is incapable of causing severe damage unless the tree is first subjected to substantial stress. Thus, keeping the tree healthy and avoiding severe stress is one important approach in preventing loss of trees to Armillaria.

Drought and leaf defoliation are two major forms of stress that favor Armillaria. In dry years it is advisable, as in all years, to make sure irrigation scheduling is appropriate. Stresses such as defoliation from persea mite, soil compaction and physical injury can exacerbate the disease. Nutrient management may minimize Armillaria effects, although little research information exists on this subject.

The second most important means of minimizing Armillaria damage is to avoid or eliminate the fungus inoculum before planting. Trees planted in former orchards will quite possibly be exposed. Since these sites cannot be avoided, here is a suggestion that will be helpful: remove stumps and old roots from the old orchard to the greatest extent possible.

 

 

armillaria
armillaria

Posted on Monday, June 10, 2013 at 7:44 AM
Tags: Armillaria (5), oak root fungus (4), root rot (21)

Armillaria Root Rot

There have been a lot of new avocado orchards planted during the last few years.  These often have been in old ‘Valencia’ orchards or lemons that had poor production.  In order to save money, growers have just cut the trees at ground level and replanted the avocados near the stumps.  Avocados have recognition of being resistant to Armillaria, but in this environment of high disease pressure, they can fail.

 

Armillaria root rot is common, yet is an infrequently identified and poorly understood disease.  It is capable of attacking most species of trees and other woody plants growing in California.  It is sometimes called “shoestring root rot” and the causal fungus is often referred to as the “honey mushroom.”  Because oak is one of the preferred hosts, it is also called “oak root fungus.”

 

If a tree undergoes a slow to rapid decline without any obvious reason, suspect Armillaria as the cause.  Certain areas, such as drainage areas from chaparral or woodlands are likely areas for this disease.  Old roots left underground provide a food base for continued fungal growth and survival.

 

General symptoms of Armillaria resemble those of other root disorders.  These symptoms are disrupted growth, yellow foliage, branch dieback, and resin or gum exudates at the root collar.  Trees may die rather abruptly without showing any decline symptoms.  Avocados typically have a rather protracted death, but in citrus it can be rapid.

 

Only rarely can the disease be diagnosed without examining the larger buttress roots and root collar of the tree.  After carefully removing the soil, examine for the presence of:

1)      Rhizomorphs, or fungal ‘shoestrings’ attached to the wood under the bark.  These may occur beneath the bark for some distance above the soil line in advanced cases, rarely they may radiate from the wood into the soil.  Rhizomorphs may also grow out from the larger roots, resembling feeder roots in appearance.  They are about the diameter of pencil lead and vary in color from black to reddish brown.  The interior consists of white mycelial tissue. 

2)      Decayed areas of wood at the root collar or on the crown roots.  Armillaria causes a white rot and the wood develops a stringy texture.  Roots in advanced stages of decay may be soft, yellowish and wet. 

3)      Veined, white mycelial fans between the bark and wood where the cambium has been killed.  Sometimes this fan (or fans) extends quite far above the soil line beneath the bark.

4)      Soil remaining attached to the roots.

5)      Characteristic mushrooms on the lower trunk or on the ground near the infected roots.  These short-lived annual fruiting structures of the disease-causing fungus may develop during the fall or winter rainy season and may occur in small clusters or in large numbers.  The stalk is typically yellow and 3 inches or more long.  Usually a ring is connected to the stalk just below the cap.  The cap is 2-5 inches across and often honey-yellow.  It may be dotted with dark brown scales.  The underside is covered with loosely spaced white or yellow gills radiating from the stem. 

 

After the disease has been identified, the grower should study the situation to determine the role Armillaria root rot has played in causing the decline or death of the tree.  Frequently the fungus is only involved in a secondary manner by invading and destroying roots after the tree has been exposed to stress of some form, such as severe drought, water logging, or soil fill over the roots.  The fungus can also act as a saprophyte feeding on dead wood.  It is frequently involved in the decay of old tree stumps and roots.

 

Many oaks are lightly infected with the disease for years with no resultant damage except for isolated pockets of buttress root rot which are walled off by the tree and have no ill effects.  Other infected trees show no damage until subjected to stress.  Accumulating evidence suggests the type of root exudate that is produced influences the susceptibility of the tree.  Certain forms of stress cause a shift in exudates that promote rapid development of the fungus and may hasten tree invasion and decay.

 

Spores are produced by the mushroom fruiting structures (mushrooms) and disseminated by air currents and introduced into new area.  Once the fungus enters the cambium and bark tissues, mycelial fans develop during the parasitic phase of the attack.  Subsequently, mycelium invades and decays the woody tissue of the roots and sometimes also the base of the trunk.  Under proper conditions the fruiting structures form at or near the base of the infected tree, completing the life cycle.

 

Direct control of the fungus in a diseased tree is not possible with present technology.  However, in many instances the fungus is incapable of causing severe damage unless the tree is first subjected to substantial stress.  Thus, keeping the tree healthy and avoiding severe stress is one important approach in preventing loss of trees to Armillaria.

 

Drought and leaf defoliation are two major forms of stress that favor Armillaria.  In dry years it is advisable, as in all years, to make sure irrigation scheduling is appropriate.  Stresses such as defoliation from persea mite, soil compaction and physical injury can exacerbate the disease.  Nutrient management may minimize Armillaria effects, although little research information exists on this subject.

 

The second most important means of minimizing Armillaria damage is to avoid or eliminate the fungus inoculum before planting.  Trees planted in former orchards will quite possibly be exposed.  Since these sites cannot be avoided, here is a suggestion that will be helpful: remove stumps and old roots from the old orchard to the greatest extent possible.

 

Below:

Armillaria mushrooms and hyphal plaques under the bark

armillaria image
armillaria image

armillaria image 2
armillaria image 2

Posted on Thursday, February 28, 2013 at 7:36 AM
Tags: armillaria (5), avocado (287), citrus (334), oak root fungus (4)
 
E-mail
 
Webmaster Email: rkrason@ucdavis.edu