Trees are living beings like you and I, which means they can get sick sometimes too. However, a diseased tree can be very dangerous because the structural integrity of the tree will be forever compromised once diseased. Tree diseases can be tree specific or affect a broad range of species, and include insect infestations or rot infections. Very often a tree will experience both rot and infestation at once. The initial cause of disease is often unknown, causing a "what came first, the chicken or the egg" scenario. Insect infestation can lead to rot by exposing the inner tissues of the tree, but insects can also be attracted to existing rot. Insect infestation has obvious signs such as borer holes, lost foliage and peeling bark. Rot however, can be completely internal and therefore unidentifiable to the untrained eye, making it the silent, more deadly killer.
So here's what you need to know about rot.
Wood decay or “rot” is inevitable. When a tree dies, it is recycled back into the earth through decay. Wood decay is the deterioration of structural wood components via decomposers, aka Fungi! Some Fungi excrete special enzymes that break down wood, absorbing and releasing the carbon and minerals that make up the wood. We have talked about fungi previously, so we know some fungi are good, while others are not. Mycorrhizal fungi in the soil is considered good fungi as it benefits tree health. Non-mycorrhizal fungi are the decomposing fungi (fig. 1) , or decomposers, and these, at least for a still standing tree, are considered bad fungi. “Bad” might be a strong word since these decomposing fungi are important to the cycle of life and matter, but these fungi do cause rot and tree disease.
Fungi is everywhere whether we like it or not, so preventing rot is impossible. Slowing down or delaying wood decay is the best we can strive for. Wood decay and rot are actually quite important to the environment and therefore to us. Rotting wood and debris supports healthy soil compositions and maintains appropriate moisture levels. Decay also contributes to the carbon cycle which is essentially the recycling of living things back into their most basic form- carbon [1]. Trees themselves are not only made up of carbon, they also “sequester” or utilize carbon through photosynthesis. Then, when a tree dies, the carbon is released by decomposers back into the environment to complete the cycle (fig 2).
While rot does have important environmental roles, it also has a significant financial impact. Wood will rot in any form, including building materials. This means that rot will not avoid your home, deck or shed. If the conditions are warm and wet, wood rot will happen anywhere. Hundreds of thousands of dollars are spent yearly to replace parts of houses and structures that have rotted away due to a bad fungal infection. Wood rot also creates mechanical failures in trees, creating significant hazards, which can cause serious damage to the trees surroundings.
ROT PROS | ROT CONS |
|---|---|
Contribute to carbon cycle | Attracts more “bad” fungi growth |
Can support diverse mycorrhiza | Creates structural failures and hazards |
Maintain moisture capacity | Spread to neighboring trees |
In order to fully understand why wood rots, we must first understand wood. (The PG kind of wood...)
**Beware, the following may induce biology class flashbacks.
Wood is made up of both non-living components and living components. The living components are specialized cells that include the parenchyma- the functional cells of the tree- as well as phloem and xylem- or the circulatory system of the tree that transport water and nutrients. Parenchyma are mainly found in the pith, which is the actively growing part of the tree (fig. 3). Phloem and xylem are found in the cambium and sapwood (fig. 3). As the sapwood dies, heartwood is formed, which makes up the rings of the tree. The nonliving parts of the tree are the cellulose and lignin, which provide structural strength. Each part of the tree contains different levels of cellulose and lignin. Decomposers break down cellulose and/or lignin, leaving behind weakened tissue.
Now we can start to understand rot.
Rot is considered a tree disease due to the fact that it weakens the tree significantly. However, not all rot is the same. Only some rot will cause tree death while others will only cause structural damage. Tree death is recognized as damage to the living cells of the tree, such as the sapwood and the vasculature (the phloem and the xylem) [2]. Rot can actually be characterized by many factors, including the type of fungi infecting the wood, the location of the rot, or the component of wood the fungi targets. Below are the most common forms of general rot found in trees:
Heart Rot
Heart rot is caused by fungus that colonizes the heartwood [3]. This type of rot does not change the physiology of the tree because only the structural components of the tree are infected. The weakened structural support leads to bole fractures which make a tree much more dangerous in high winds.
Heart rot fungi infect trees through contact and release of spores during bole fractures (fig. 4). This is very similar to how the flu is spread between people. If you are sick and touch another person or sneeze on them, germs spread making more and more people sick. When an infected tree has a bole failure (aka falls over) neighboring trees are wounded by the infected tree during the fall. The exposed wound on the neighboring tree is now infected with fungal spores, which will gradually spread throughout the specimen until bole failure, repeating the infection cycle [2].
There are 3 forms of heart rot; brown, red, and white.
Brown rot
->Breaks down cellulose,
->Signs: Large wounds; brown stained heartwood that is disintegrating into fragile cubes.
White rot
->Breaks down both cellulose and lignin.
->Signs: spalted wood; white zone lines; and moist, spongey pockets of decay.
**Fun fact- spalted wood has become a hot commodity for wood workers for the interesting caverns that the fungus creates inside the wood. So sometimes decayed wood can still be useful!
Red Rot
->Breaks down lignin
->Signs: terminal dieback in foliage; soft, red stained heartwood; and leathery red fruiting bodies.
Soft Rot
Soft rot is typically a combination of both fungus and bacteria [3]. Trees with structural deformities such as eutypella or previously healed injury seams are susceptible to soft rot since the inner tissues remain slightly exposed. This exposed tissue will hold moisture due to the lack of external protection, creating a breeding ground for bacteria and soft rot fungi. Most often, soft rot infection will spread from tree to tree via wind or animal/insect carriers, but can spread via contact as well. Once infected, the tree's cellulose adjacent to the infection is broken down. As a result, trees will develop isolated structural damage which can lead to limb failure. In extreme cases, both the heartwood and sapwood will be damaged, causing death to the tree via destruction of the cell walls of the living cells [4]. Soft rot commonly infects non-living trees and wood as well, meaning this type of rot is often a secondary infection to an already declining tree.
->Signs- hollow holes and cavities; foliage dieback.
Root Rot
Root rot is the progressive destruction of rootlets, coarse roots and collar of the tree (fig. 5). Root rot is caused by pathogenic soil fungi. While these fungi are found in the soil, they are non-mycorrhizal, meaning that they inhibit good mycorrhizal fungal growth. These pathogens release nitrogen, which reduce mycorrhiza in the surrounding areas, allowing more pathogens to grow and infect roots [5]. Root rot is becoming more and more prevalent due to the reduced winter dormancy periods as a result of rising global temperatures. The cold temperatures of winter typically kill off any soil pathogens. With spring warming happening earlier and earlier each year, the pathogens have longer to repopulate, creating populations that the trees and good mycorrhiza cannot withstand. Root rot is a serious concern for farmers since a reduced yield due to rot will directly impact the profit the field produces. In trees, root rot prevents conduction of water and nutrients, leading to tree death. Scientists call this loss of conduction “tyloses”, which is essentially a balloon-like growth within the phloem and xylem in the sapwood that clogs the vessels. Trees with root rot will become significant hazards because of the loss of living tissue, but also because of the loss of support from the root system. The roots provide most of the stability for a tree to stay standing, anchoring the tree in the soil. Once that anchor becomes compromised, there is little stopping that tree from uprooting. The best way to prevent root rot is to encourage good mycorrhizal fungi growth to outcompete the root rot fungi, as well as ensuring proper drainage to avoid the soil being too wet. The trees most commonly infected with root rot are the deciduous (leafy) trees, such as oak, beech, birch and maple trees.
->Signs- bark necrosis; branch deterioration; yellowing foliage and crown dieback.
**Fun fact- the amount of crown dieback is equal to the amount of root dieback. So if 50% of the leaves on your tree have died, it is safe to say that 50% of the roots have also died.
Butt Rot
Butt rot is obviously characterized by the location in the tree that the fungi targets- the butt. (Stop snickering!) Butt rot is similar to root rot in that it is spread and harbored in the roots, but can also spread via the fungal spores in the air. Once the tree is infected, the fungi attacks the heartwood and sapwood in the lower section of the trunk. You might be thinking-”but why does it only infect the butt?” The infection in the butt of the tree causes a barreling or swelling effect that inhibits all conduction to the rest of the tree. Trees with butt rot are often infected with heart rot or soft rot as well due to the reduced health of the tree from the initial butt rot infection. However, the butt rot fungi are only ever identified within the collar or butt of the tree. Conifer trees are the most common tree to be infected with butt rot. In the forestry industry, butt rot has taken a serious toll on the volume of logs harvested. In Europe, Norway spruce account for 73% of roundwood harvested, but for the logs to be acceptable, they must be "butt rot free". (I think we can all agree that being "butt rot free" is the way to be.) In the last several years, the European logging industry has seen a 48% loss in log volume [6], and an 800 million Euro annual loss from butt rot alone [7].
->Signs- little to no foliage, cankers on lower trunk, swelling/barreling in the butt.
So there you have it, a Coles-notes guide to general tree rot diseases. Rot is a big topic, with lots of intricacies and specifics. We only covered the general diseases here. There are hundreds of rots that only infect specific trees, such as the fungi that causes Beech Bark Disease or Dutch Elm Disease. However, because rot and fungi are everywhere, we wanted to share the general information for everyone to be able to identify when a tree is becoming a hazard. Remember, not all fungi are bad, but fungi that cause rot are bad for your tree and its surroundings.
References
Schwarze, F. (2007) Wood decay under the microscope. Fungal Biology Reviews. https://doi.org/10.1016/j.fbr.2007.09.001
Hennon, P.E. (1995) Are heart rot fungi major factors of disturbance in the gap-dynamic forests? Northwest Science, 69(4).
Jha, S.K. (2020) Identification and management of heart-rot fungi. Banko Janakari, 30:2. DOI:10.3126/banko.v30i2.33482
Bari, E., et al. (2020) Comparison of the Decay Behavior of Two White-Rot Fungi in Relation to Wood Type and Exposure Conditions. Microorgamisms, 8(12):1931.
doi: 10.3390/microorganisms8121931.
Jung, T., and Blaschke, H. (1996) Phytophthora root rot in declining forest trees. Phyton-International Journal of Experimental Botany, 36:95-102.
Noordermeer, L., et al. (2023) Econmic losses caused by butt rot in Norway Spruce trees in Norway. Scandanavian Journal of Forest Research, 38(7-8): 497-505. doi.org/10.1080/02827581.2023.2273252
Asiegbu, F.O., Adomas, A., and Stenlid, J. (2005) Conifer root and butt rot cause by Heterobasidion annosum (Fr.) Bref. s.l. Molecular Plant Pathology, 6(4): 395-409. DOI: 10.1111/j.1364-3703.2005.00295.x
Admire your blog so much - really well written and very readable. Keep it up!