Fungus is typically associated with dirty, bad things. Rarely do you hear someone say, “Oh good it has fungus on it”. Ok probably never. Of late however, significant research has been done which proves at least some fungi are beneficial to trees.
First the good bit, info about fungi and your tree.
Sorry in advance for the large science geek wording but here it goes. Fungi is omnipresent in our world. Fungi makes up a large portion of the Earth's microbial colonization, which inhabits everything; soil, water, the air, even our skin. The fungal colonization and interaction with trees and plants is a topic which has been extensively studied. We are learning that the relationship between a plant, its root system and the fungi in the soil is vital to its health.
Fungal networks, referred to as mycorrhizae, extend underground between roots (figure 1). Ectomycorrhiza densely sheath roots and are connected by hyphae, which are hair-like extensions. Endomycorrhiza, or arbuscular mycorrhizae, are embedded in the root tissue. Together, both types of mycorrhizal fungi act in symbiosis with the host plant or tree. Symbiosis suggests that the association is positive for both species. For example, the fungal network increases the surface area of the root system, allowing for additional nutrient cycling and water uptake. Minerals such as carbon, nitrogen, phosphorus, and iron are funneled bidirectionally from nutrient rich areas of the soil back to the roots, which are considered nutrient depleted. 65% of a tree's nutrients are provided via the mycorrhizae, suggesting a certain level of dependency [1]. This increased nutrient uptake also leads to improved plant fitness and better stress resistance in plants. Mycorrhizal fungal species naturally produce growth hormones and molecules which directly benefit the host (i.e. cytokines are secreted, which improve plant growth and aid in fighting disease). Chemical signals are also passed via the fungal network, acting as a communication method for triggering reproduction and defense signals. In return, the host plant provides the fungi with glucose as nutrients, which is only produced during photosynthesis. So long story short, when talking about roots, “Oh good, it has fungus on it!”.
This is fun, let's keep going. While both morphologies of mycorrhizae participate in the symbiotic relationship with host plants, endomycorrhiza are more geographically ubiquitous and abundant. 80% of plants, including trees, have endomycorrhiza embedded in their roots [2]. Some host species have dependency on different endomycorrhizal species. For example, leafy deciduous trees, or angiosperms (I did apologize in advance for geeky wording), are mostly associated with Diaporthole fungi, while conifer trees, or gymnosperms, are mostly associated with Helotide fungi [3]. Ancestral studies of mycorrhizal fungi show the first divergence of this specific host-fungi interaction was predicted to be 300 million years ago [3]. At that time in early Earth, plants were moving from water, where there is an abundance of organic matter and nutrients, to land, which is much more depleted in comparison. Mycorrhizal fungi were maintained in the transfer to land, suggesting that the symbiosis between the fungi and the plants is evolutionary as well as necessary for successful growth.
Now the bad and the ugly bits.
Despite the overwhelming results proving that fungi is necessary for healthy, happy trees, there are times that fungus does not benefit the host. Fungi in the soil associated with roots are beneficial. However, fungal fruiting bodies seen on bark and limbs is a sign of fungal infection, which is bad news for the tree. For a tree to become infected with a fungus and for fruiting bodies to grow, the fungus must first invade the tree's tissues. The fungus secretes enzymes which aid in its growth. The enzymes also cause significant damage to the internal tissues of the tree, compromising structure and health. These fungal infections are often a secondary reaction to an already dead, dying or diseased tree. Healthy trees with appropriate mycorrhizae association have natural defense to prevent fungal infections or disease.
What does this all mean to you and the trees you love?
First of all, watch for fungi on the trunk and branches of your trees. This can be a sign that your tree is in trouble and may become a danger to you and your property.
Beyond that, the biggest question regarding trees and fungus, is how can we as humans use this knowledge? Biofertilizers are the answer. Biofertilizers, or mycorrhizal inoculants, are an alternative to chemical fertilizers, which leach into water systems and have significant negative effects on the environment. Inoculants supply soil with specifically chosen fungi which supports a beneficial mycorrhizal network, and resultantly, healthy plants. The inoculation process is still relatively new and studies are still determining the best method. Issues arise in choosing the best species of endo- and ecto- mycorrhizal fungi to inoculate. Fungi inoculated from nursery plants have been proven to be outcompeted by native fungi once introduced to natural environments, becoming essentially obsolete [4]. Even if the best species are identified, inoculation and production in a lab, on mass, has been proven to produce fungi that perform differently than those inoculated from natural sources [4]. In spite of these challenges in commercializing and utilizing the benefits of mycorrhizae, the biofertilizers currently on the market are still far out performing chemical fertilizers in terms of plant health and environmental impact.
So, next time you hear “fungi”, don’t immediately think “bad”. Think about how fundamentally important fungus is to trees, which are fundamentally important to us!
Foot Notes
Lagomarsino, V. (2019). "Exploring The Underground Network of Trees – The Nervous System of the Forest." Harvard University, SITNBoston. Exploring The Underground Network of Trees – The Nervous System of the Forest - Science in the News (harvard.edu). Received January 10, 2024.
Kheyrodin, H. (2014). "Plant and Soil relationship between Fungi." International Journal of Research in Biosciences, Vol 2:9. Pages 42-49.
Sieber, T.N. (2007). "Endophytic fungi in forest trees: are they mutualistic?" Fungal Biology reviews 21, 2007. Pages 75-89.
Jumpponen, A., et al. (2004). "Ecological relationships among hypogenous fungi and trees: interferences from association analysis integrated with habitat modeling." Mycologia, 96:3. Pages 510-525.
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